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Definitions


Sexual intercourse may be defined by different words, including coitus, copulation, coition, or intercourse (which is typically shorthand for sexual intercourse). The term coitus is derived from the Latin word coitio or coire, meaning "a coming together or joining together" or "to go together", and it describes a variety of sexual activities under ancient Latin names, but usually refers exclusively to penile–vaginal penetration. This is often termed vaginal intercourse or vaginal sex. The term vaginal sex, and less often vaginal intercourse, may also refer to any vaginal sexual activity, particularly if penetrative, including sexual activity between lesbian couples. Copulation, by contrast, more often refers to the mating process, especially for non-human animals; it can denote any sexual activity between opposite-sex or same-sex pairings, but is generally defined as the sexually reproductive act of transferring sperm from a male to a female or sexual procreation between a man and a woman.

Although the terms sexual intercourse and sex most commonly denote penile–vaginal intercourse,
 * See page 11 onwards and pages 47–49 for views on what constitutes virginity loss and therefore sexual intercourse or other sexual activity; source discusses how gay and lesbian individuals define virginity loss, and how the majority of researchers and heterosexuals define virginity loss/"technical virginity" by whether or not a person has engaged in penile–vaginal sex.
 * sex and the phrase "have sex" can mean any penetrative or non-penetrative sexual activity between two or more people. The World Health Organization (WHO) states that non-English languages and cultures "use different terms for sexual activity, with slightly different meanings". Various vulgar or slang words and euphemisms are also used to describe sexual intercourse or other sexual activity, such as the term fuck, shag, and the phrase "sleep together". Penetration of the vagina by the erect penis is additionally known as intromission, or by the Latin name immissio penis (Latin for "insertion of the penis").
 * See page 11 onwards and pages 47–49 for views on what constitutes virginity loss and therefore sexual intercourse or other sexual activity; source discusses how gay and lesbian individuals define virginity loss, and how the majority of researchers and heterosexuals define virginity loss/"technical virginity" by whether or not a person has engaged in penile–vaginal sex.
 * sex and the phrase "have sex" can mean any penetrative or non-penetrative sexual activity between two or more people. The World Health Organization (WHO) states that non-English languages and cultures "use different terms for sexual activity, with slightly different meanings". Various vulgar or slang words and euphemisms are also used to describe sexual intercourse or other sexual activity, such as the term fuck, shag, and the phrase "sleep together". Penetration of the vagina by the erect penis is additionally known as intromission, or by the Latin name immissio penis (Latin for "insertion of the penis").
 * sex and the phrase "have sex" can mean any penetrative or non-penetrative sexual activity between two or more people. The World Health Organization (WHO) states that non-English languages and cultures "use different terms for sexual activity, with slightly different meanings". Various vulgar or slang words and euphemisms are also used to describe sexual intercourse or other sexual activity, such as the term fuck, shag, and the phrase "sleep together". Penetration of the vagina by the erect penis is additionally known as intromission, or by the Latin name immissio penis (Latin for "insertion of the penis").
 * sex and the phrase "have sex" can mean any penetrative or non-penetrative sexual activity between two or more people. The World Health Organization (WHO) states that non-English languages and cultures "use different terms for sexual activity, with slightly different meanings". Various vulgar or slang words and euphemisms are also used to describe sexual intercourse or other sexual activity, such as the term fuck, shag, and the phrase "sleep together". Penetration of the vagina by the erect penis is additionally known as intromission, or by the Latin name immissio penis (Latin for "insertion of the penis").

Vaginal, anal and oral sex are recognized as sexual intercourse more often than other sexual behaviors. While non-penetrative and non-penile–vaginal sexual activities may be regarded as sexual intercourse, they might alternatively be considered a means of maintaining virginity (sometimes called "technical virginity)" or labeled "outercourse", regardless of any penetrative aspects; this is more often the case for oral sex than for anal sex.
 * See page 11 onwards and pages 47–49 for views on what constitutes virginity loss and therefore sexual intercourse or other sexual activity; source discusses how gay and lesbian individuals define virginity loss, and how the majority of researchers and heterosexuals define virginity loss/"technical virginity" by whether or not a person has engaged in penile–vaginal sex.
 * Virginity loss is often based on penile–vaginal intercourse partly because heterosexual couples may engage in anal or oral sex not only for sexual pleasure, but as ways of maintaining that they are virgins if they have not engaged in the reproductive act of coitus. Similarly, some gay men consider frotting or oral sex as ways of maintaining their virginities, with penile-anal penetration defined as sexual intercourse and resulting in virginity loss, while other gay men may define frotting or oral sex as their main forms of sexual activity. Lesbians may define oral sex or fingering as sexual intercourse and subsequently an act of virginity loss,  or tribadism as a primary form of sexual activity.
 * See page 11 onwards and pages 47–49 for views on what constitutes virginity loss and therefore sexual intercourse or other sexual activity; source discusses how gay and lesbian individuals define virginity loss, and how the majority of researchers and heterosexuals define virginity loss/"technical virginity" by whether or not a person has engaged in penile–vaginal sex.
 * Virginity loss is often based on penile–vaginal intercourse partly because heterosexual couples may engage in anal or oral sex not only for sexual pleasure, but as ways of maintaining that they are virgins if they have not engaged in the reproductive act of coitus. Similarly, some gay men consider frotting or oral sex as ways of maintaining their virginities, with penile-anal penetration defined as sexual intercourse and resulting in virginity loss, while other gay men may define frotting or oral sex as their main forms of sexual activity. Lesbians may define oral sex or fingering as sexual intercourse and subsequently an act of virginity loss,  or tribadism as a primary form of sexual activity.
 * Virginity loss is often based on penile–vaginal intercourse partly because heterosexual couples may engage in anal or oral sex not only for sexual pleasure, but as ways of maintaining that they are virgins if they have not engaged in the reproductive act of coitus. Similarly, some gay men consider frotting or oral sex as ways of maintaining their virginities, with penile-anal penetration defined as sexual intercourse and resulting in virginity loss, while other gay men may define frotting or oral sex as their main forms of sexual activity. Lesbians may define oral sex or fingering as sexual intercourse and subsequently an act of virginity loss,  or tribadism as a primary form of sexual activity.

Researchers commonly define sexual intercourse as penile–vaginal intercourse while using specific terms, such as anal sex or oral sex, for other sexual behaviors.
 * See page 11 onwards and pages 47–49 for views on what constitutes virginity loss and therefore sexual intercourse or other sexual activity; source discusses how gay and lesbian individuals define virginity loss, and how the majority of researchers and heterosexuals define virginity loss/"technical virginity" by whether or not a person has engaged in penile–vaginal sex.
 * Scholars Richard M. Lerner and Laurence Steinberg state that researchers also "rarely disclose how they define sex or even whether they resolved potential discrepancies in definitions of sex". Learner and Steinberg attribute researchers' focus on penile–vaginal sex to "the larger culture's preoccupation with this form of sexual activity," and have expressed concern that the "widespread, unquestioned equation of penile–vaginal intercourse with sex reflects a failure to examine systematically 'whether the respondent's understanding of the question [about sexual activity] matches what the researcher had in mind'". This focus can also relegate other forms of mutual sexual activity to foreplay or contribute to them not being regarded as "real sex", and limit how rape is defined.   It may also be that conceptually conflating sexual activity with vaginal intercourse and sexual function hinders and limits information about sexual behavior that non-heterosexual people may be engaging in, or information about heterosexuals who may be engaging in non–vaginal sexual activity.
 * See page 11 onwards and pages 47–49 for views on what constitutes virginity loss and therefore sexual intercourse or other sexual activity; source discusses how gay and lesbian individuals define virginity loss, and how the majority of researchers and heterosexuals define virginity loss/"technical virginity" by whether or not a person has engaged in penile–vaginal sex.
 * Scholars Richard M. Lerner and Laurence Steinberg state that researchers also "rarely disclose how they define sex or even whether they resolved potential discrepancies in definitions of sex". Learner and Steinberg attribute researchers' focus on penile–vaginal sex to "the larger culture's preoccupation with this form of sexual activity," and have expressed concern that the "widespread, unquestioned equation of penile–vaginal intercourse with sex reflects a failure to examine systematically 'whether the respondent's understanding of the question [about sexual activity] matches what the researcher had in mind'". This focus can also relegate other forms of mutual sexual activity to foreplay or contribute to them not being regarded as "real sex", and limit how rape is defined.   It may also be that conceptually conflating sexual activity with vaginal intercourse and sexual function hinders and limits information about sexual behavior that non-heterosexual people may be engaging in, or information about heterosexuals who may be engaging in non–vaginal sexual activity.

Studies regarding the definition of sexual intercourse sometimes conflict. A 1999 study by the Kinsey Institute examined the definition of sex based on a 1991 random sample of 599 college students from 29 U.S. states; it reported that while "virtually every college student they surveyed considered penile–vaginal intercourse to be sex," and 19–20% said that anal intercourse was not sex, 60% said oral-genital contact (fellatio, cunnilingus) did not constitute having sex. Similarly, a 2003 study published in the Canadian Journal of Human Sexuality focusing on definitions of having sex and noting studies concerning university students from the United States, the United Kingdom, and Australia reported that "while the vast majority of respondents (more than 97%) in these three studies included penile–vaginal intercourse in their definition of sex, fewer (between 70% and 90%) respondents considered penile-anal intercourse to constitute having sex" and that "oral-genital behaviours were defined as sex by between 32% and 58% of respondents". The Centers for Disease Control and Prevention (CDC) stated in 2009 that "although there are only limited national data about how often adolescents engage in oral sex, some data suggest that many adolescents who engage in oral sex do not consider it to be 'sex'; therefore they may use oral sex as an option to experience sex while still, in their minds, remaining abstinent".

The specificity of questions concerning sexual activity can additionally affect definitions of sexual intercourse or other sexual behaviors. Another study by the Kinsey Institute sampled 484 people, ranging in ages 18–96. The study reported that nearly 95% of its participants "agreed that penile–vaginal intercourse meant 'had sex.' But the numbers changed as the questions got more specific". 11% of respondents based "had sex" on whether the man had achieved an orgasm, concluding that absence of an orgasm does not constitute "having had" sex; "about 80 percent of respondents said penile-anal intercourse meant 'had sex.' About 70 percent of people believed oral sex was sex." Condom use is also a factor, with some men stating that sexual activity involving the protection of a condom is not "real sex" or "the real thing". One study reported that older generations of men (65 or older) in particular do not view sexual activity involving the protection of a condom to be sex. This view is common among men in Africa, where sexual activity involving the protection of a condom is often associated with emasculation because condoms prevent direct penile–to–skin genital contact.

Bonding
Copulation ranges from a purely reproductive activity to one of emotional bonding. For example, sexual intercourse and sexual activity in general often play a strong role in human bonding. In many societies, it is normal for couples to have sexual intercourse while using some method of birth control (contraception), sharing pleasure and strengthening their emotional bond through sexual activity even though they are deliberately avoiding pregnancy.

In humans and bonobos, the female undergoes relatively concealed ovulation so that male and female partners commonly do not know whether she is fertile at any given moment. One possible reason for this distinct biological feature may be formation of strong emotional bonds between sexual partners important for social interactions and, in the case of humans, long-term partnership rather than immediate sexual reproduction. For humans in particular, cooperative behavior in a community and, by extension, sexual activity reinforce social bonds between individuals and may form larger social structures. The resulting cooperation encourages collective tasks that promote the survival of each member of the group.

Stimulation
Sexual intercourse or other sexual activity can encompass various sexually stimulating factors (physiological stimulation or psychological stimulation), including different sex positions or the use of sex toys. Foreplay may precede some sexual activities, often leading to sexual arousal of the partners and resulting in the erection of the penis or |natural lubrication of the vagina. It is also common for people to be as sexually satisfied by being kissed, touched erotically, or held as they are by sexual intercourse.

In most mammals, the anatomy of the reproductive organs and some circuits of the nervous system are specifically organized for heterosexual copulation. Copulation corresponds to a succession of innate sexual reflexes: erection, vaginal lubrication, lordosis behavior, immobilization, intromission, pelvic thrust and ejaculation. These copulatory reflexes are controlled by hormones and facilitated by sexual pheromones. In hominidae, including humans, several biological factors that control copulation are modified. These changes in neurobiological control of copulation are particularly evident in females of hominidae. Non-primate females copulate only when in estrus, but sexual intercourse is possible at any time of the menstrual cycle for women. Sex pheromones facilitate copulatory reflexes, but, in humans, the detection of pheromones is impaired and they have only residual effects. Non-primate females put themselves in the crucial lordosis position and remain motionless, but these motor copulatory reflexes are no longer functional in women. Unlike mammalian copulation, where copulation is a reflex, controlled by hormones and influenced by pheromones, human sexual intercourse is mostly a voluntary and learned activity, in order to obtain genital sexual rewards.

During coitus, the partners orient their hips to allow the penis to move back and forth in the vagina to cause friction, typically without fully removing the penis. In this way, they stimulate themselves and each other, often continuing until orgasm in either or both partners is achieved.

For human females, stimulation of the clitoris plays a significant role in sexual activity; 70–80% of women require direct clitoral stimulation to achieve orgasm,  though indirect clitoral stimulation (for example, via vaginal intercourse) may also be sufficient (see orgasm in females). Because of this, some couples may engage in the woman on top position or the coital alignment technique, a technique combining the "riding high" variation of the missionary position with pressure-counterpressure movements performed by each partner in rhythm with sexual penetration, to maximize clitoral stimulation. Anal sex involves stimulation of the anus, anal cavity, sphincter valve or rectum; it most commonly means the insertion of a man's penis into another person's rectum, but may also mean the use of other sex toys or fingers to penetrate the anus, pegging, or anilingus.

Oral sex consists of all the sexual activities that involve the use of the mouth and throat to stimulate genitalia or anus. It is sometimes performed to the exclusion of all other forms of sexual activity, and may include the ingestion or absorption of semen or vaginal fluids.

Fingering (or digital penetration or digital intercourse) involves the manual manipulation of the clitoris, rest of the vulva, vagina or anus for the purpose of sexual arousal and sexual stimulation; it may constitute the entire sexual encounter or it may be part of mutual masturbation, foreplay or other sexual activities.

Reproduction
Sexual intercourse is perhaps 385 million years old, and it is likely that the oldest jawed fish on Earth was the first animal to reproduce by copulation. Reproduction among humans usually occurs with penile–vaginal penetration. Male orgasm usually includes ejaculation, a series of muscular contractions that deliver semen containing male gametes known as sperm cells or spermatozoa from the penis into the vagina. The subsequent route of the sperm from the vault of the vagina is through the cervix and into the uterus, and then into the fallopian tubes. Millions of sperm are present in each ejaculation, to increase the chances of one fertilizing an egg or ovum (see sperm competition). When a fertile ovum from the female is present in the fallopian tubes, the male gamete joins with the ovum, resulting in fertilization and the formation of a new embryo. When a fertilized ovum reaches the uterus, it becomes implanted in the lining of the uterus (the endometrium) and a pregnancy begins. Unlike most species, human sexual activity is not linked to periods of estrus and can take place at any time during the reproductive cycle, even during pregnancy.

When a sperm donor has sexual intercourse with a woman who is not his partner and for the sole purpose of impregnating the woman, this may be known as natural insemination, as opposed to artificial insemination. Artificial insemination is a form of assisted reproductive technology, which are methods used to achieve pregnancy by artificial or partially artificial means. For artificial insemination, sperm donors may donate their sperm through a sperm bank, and the insemination is performed with the express intention of attempting to impregnate the female; to this extent, its purpose is the medical equivalent of sexual intercourse.

Reproductive methods also extend to gay and lesbian couples. For gay male pairings, there is the option of surrogate pregnancy; for lesbian couples, there is donor insemination in addition to choosing surrogate pregnancy. Surrogacy and donor insemination remain the primary methods. Surrogacy is an arrangement in which a woman carries and delivers a child for another couple or person. The woman may be the child's genetic mother (traditional surrogacy) or she may carry a pregnancy to delivery after having another woman's eggs transferred to her uterus (gestational surrogacy). Gay or lesbian pairings who want the host to have no genetic connection to the child may choose gestational surrogacy and enter into a contract with an egg donor. Gay male couples might decide that they should both contribute semen for an in vitro fertilisation (IVF) process, which further establishes the couple's joint intention to become parents. Lesbian couples often have contracts drafted to extinguish the legal rights of the sperm donor, while creating legal rights for the parent who is not biologically related to the child.

Safe sex and birth control
There are a variety of safe sex methods that are practiced by heterosexual and same-sex couples, including non-penetrative sex acts, and heterosexual couples may use oral or anal sex (or both) as a means of birth control. However, pregnancy can still occur with anal sex or other forms of sexual activity if the penis is near the vagina (such as during intercrural sex or other genital-genital rubbing) and its sperm is deposited near the vagina's entrance and travels along the vagina's lubricating fluids; the risk of pregnancy can also occur without the penis being near the vagina because sperm may be transported to the vaginal opening by the vagina coming in contact with fingers or other non-genital body parts that have come in contact with semen.

Safe sex is a relevant harm reduction philosophy, and condoms are used as a form of safe sex and contraception. Condoms are widely recommended for the prevention of sexually transmitted infections (STIs). According to reports by the National Institutes of Health (NIH) and World Health Organization (WHO), correct and consistent use of latex condoms reduces the risk of HIV/AIDS transmission by approximately 85–99% relative to risk when unprotected. The most effective way to avoid sexually transmitted infections is to abstain from sexual intercourse, especially vaginal, anal, and oral sexual intercourse.

Decisions and options concerning birth control can be affected by cultural reasons, such as religion, gender roles or folklore. In the predominantly Catholic countries Ireland, Italy and the Philippines, fertility awareness and the rhythm method are emphasized while disapproval is expressed with regard to other contraceptive methods. Worldwide, sterilization is a more common birth control method, and use of the intrauterine device (IUD) is the most common and effective way of reversible contraception. Conception and contraception are additionally a life-and-death situation in developing countries, where one in three women give birth before age 20; however, 90% of unsafe abortions in these countries could be prevented by effective contraception use.

In 2004, the Guttmacher Institute indicated in 2002 that 62% of the 62 million women aged 15–44 are currently using a contraceptive method, that among U.S. women who practice contraception, the birth-control pill is the most popular choice (30.6%), followed by tubal sterilization (27.0%) and the male condom (18.0%), and that 27% of teenage women using contraceptives choose condoms as their primary method. A 2006 Kaiser Family Foundation report stated that among sexually active 15- to 19-year-olds in the U.S., 83% of females and 91% of males reported using at least one method of birth control during last intercourse.

The National Survey of Sexual Health and Behavior (NSSHB) indicated in 2010 that "1 of 4 acts of vaginal intercourse are condom-protected in the U.S. (1 in 3 among singles)," that "condom use is higher among black and Hispanic Americans than among white Americans and those from other racial groups," and that "adults using a condom for intercourse were just as likely to rate the sexual extent positively in terms of arousal, pleasure and orgasm than when having intercourse without one".

Prevalence
Penile–vaginal penetration is the most common form of sexual intercourse. Studies indicate that most heterosexual couples engage in vaginal intercourse nearly every sexual encounter. The National Survey of Sexual Health and Behavior (NSSHB) reported in 2010 that vaginal intercourse is "the most prevalent sexual behavior among men and women of all ages and ethnicities". In 2013, Clint E. Bruess et al. stated that it "is the most frequently studied behavior" and, besides in many cultures being what is usually meant when people refer to "having sex" or "sexual intercourse", is "often the focus of sexuality education programming for youth."

Regarding oral or anal intercourse, the CDC stated in 2009, "Studies indicate that oral sex is commonly practiced by sexually active male-female and same-gender couples of various ages, including adolescents." The 2010 NSSHB study reported that vaginal intercourse was practiced more than insertive anal intercourse among men, but that 13% to 15% of men aged 25 to 49 practiced insertive anal intercourse. Receptive anal intercourse was infrequent among men, with approximately 7% of men aged 14 to 94 years old having said that they were a receptive partner during anal intercourse. It said that women engage in anal intercourse less commonly than men, but that the practice is not uncommon among women; it was estimated that 10% to 14% of women aged 18 to 39 years old practiced anal sex in the past 90 days, and that most of the women said they practiced it once a month or a few times a year.

The prevalence of sexual intercourse has been compared cross-culturally. In 2003, Michael Bozon of the French Institut national d'études démographiques conducted a cross-cultural study titled "At what age do women and men have their first sexual intercourse?". In the first group of the contemporary cultures he studied, which included sub-Saharan Africa (listing Mali, Senegal and Ethiopia), the data indicated that the age of men at sexual initiation in these societies is at later ages than that of women, but is often extra-marital; the study considered the Indian subcontinent to also fall into this group, though data was only available from Nepal.

In the second group, the data indicated families encouraged daughters to delay marriage, and to abstain from sexual activity before that time. However, sons are encouraged to gain experience with older women or prostitutes before marriage. Age of men at sexual initiation in these societies is at lower ages than that of women; this group includes Latin cultures, both from southern Europe (Portugal, Greece and Romania are noted) and from Latin America (Brazil, Chile, and the Dominican Republic). The study considered many Asian societies to also fall into this group, although matching data was only available from Thailand.

In the third group, age of men and women at sexual initiation was more closely matched; there were two sub-groups, however. In non-Latin, Catholic countries (Poland and Lithuania are mentioned), age at sexual initiation was higher, suggesting later marriage and reciprocal valuing of male and female virginity. The same pattern of late marriage and reciprocal valuing of virginity was reflected in Singapore and Sri Lanka. The study considered China and Vietnam to also fall into this group, though data were not available. In northern and eastern European countries, age at sexual initiation was lower, with both men and women involved in sexual intercourse before any union formation; the study listed Switzerland, Germany and the Czech Republic as members of this group.

Concerning United States data, national surveys in 1995 indicated that at least 3/4 of all men and women in the U.S. engaged in sexual intercourse by their late teenage years, and more than 2/3 of all sexually experienced teens had two or more partners. Based on the 2002 National Survey of Family Growth, published by the U.S. Department of Health and Human Services, the average age of first sexual intercourse in U.S. participants aged 15 to 44 was 17.3 years for females and 17.0 years for males. Special tabulations by the National Center for Health Statistics suggest that this figure changed between 2006 and 2010 to 17.1 years for both males and females. The Centers for Disease Control and Prevention (CDC) stated that 45.5 percent of girls and 45.7 percent of boys had engaged in sexual activity by 19 in 2002; in 2011, reporting their research from 2006–2010, they stated that 43% of American unmarried teenage girls and 42% of American unmarried teenage boys have ever engaged in sexual intercourse. The CDC also reports that American girls will most likely lose their virginity to a boy who is 1 to 3 years older than they are.

The U.S. Department of Health and Human Services reported in 2002 that teenagers are delaying sexual intercourse and other sexual activity until older ages. Between 1988 and 2002, the percentage of people in the U.S. who had sexual intercourse between the ages of 15 to 19 fell from 60 to 46 percent for never-married males, and from 51 to 46 percent for never-married females. A 2008 survey conducted by YouGov for Channel 4 suggested that 40% of all 14 to 17-year-olds are sexually active, 74% of sexually active 14 to 17-year-olds have had a sexual experience under the age of consent, and 6% of teens would wait until marriage before having sex.