Sexual intercourse

• 1. Introduction

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  Sexual intercourse is one of the few human behaviors that researchers study constantly yet rarely agree on how to define. A married couple may insist they remain virgins. A court in New Hampshire once ruled that two women in a sexual relationship had not committed adultery. The Federal Bureau of Investigation, until 2012, counted rape as a crime only men could commit against women. The same physical act carries a dozen contested meanings depending on who is asking, why, and in which century. So what actually counts as sex? Why does so much hinge on whether a penis enters a vagina, and what gets erased when that single act becomes the measure of everything? The answers reach into Latin grammar, the reproductive habits of dragonflies, the texts of half a dozen faiths, and the bedrooms of couples trying to time conception to the day.

• 2. The Trouble With Definitions

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  Coitus comes from the Latin coire, meaning to go together or to come together, and it usually points to one specific act, penile-vaginal penetration. That narrowness has consequences. Vaginal, anal, and oral sex are recognized as sexual intercourse more often than other behaviors, with oral sex ranking lowest of the three. Heterosexual couples sometimes engage in anal or oral sex while maintaining they are still virgins, because they have not performed the reproductive act of coitus. Some gay men treat frot or oral sex as ways of keeping their virginity, reserving penile-anal penetration as the act that ends it. Lesbians may count oral sex, fingering, or tribadism as intercourse and therefore as virginity loss. Scholars Richard M. Lerner and Laurence Steinberg observe that researchers rarely disclose how they define sex, or whether they ever resolved the discrepancies in their definitions. The two attribute the focus on penile-vaginal sex to what they call the larger culture's preoccupation with this form of sexual activity. That focus, they warn, demotes other forms of mutual sex to mere foreplay, and it narrows what the law is willing to call rape. The Centers for Disease Control and Prevention noted that many adolescents who have oral sex do not consider it sex, and so treat it as a way to experience sex while, in their own minds, staying abstinent. Upton and colleagues added a worry that follows from this. People who do not classify an act as sex may also fail to connect it to the health risks it carries.

• 3. The Body's Mechanics

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  Seventy to eighty percent of women require direct clitoral stimulation to reach orgasm, a fact that sits awkwardly against the physical structure of coitus. The missionary position is the most common human sex position, yet it favors penile stimulation over the clitoris, whose location usually requires manual or oral attention for a woman to climax. Some couples respond with the coital alignment technique, which combines a riding-high variation of the missionary position with rhythmic pressure-counterpressure movements to maximize clitoral contact. Human bodies have shed much of the machinery other species rely on. Non-primate females copulate only when in estrus, but a woman can have intercourse at any point in her menstrual cycle. Those females put themselves in the lordosis position and hold still, a motor reflex no longer functional in women. Sex pheromones drive copulatory reflexes across many organisms, yet in humans their detection is impaired and leaves only residual effects. During coitus the partners angle their hips so the penis moves back and forth without fully withdrawing, generating the friction that stimulates both until orgasm is reached in one or both. Touch alone can carry the same weight. People are often as satisfied by being kissed, held, or erotically touched as they are by intercourse itself.

• 4. Conception And Its Workarounds

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  Millions of sperm are ejaculated to raise the odds of fertilization, though a single one is enough to fertilize an ovum. The sperm travels through the vaginal vault, the cervix, the uterus, and into the fallopian tubes, where a fertile egg may wait. Pregnancy itself begins only after the fertilized ovum implants in the endometrium, the lining of the uterus. Timing matters intensely. Pregnancy rates peak in the window from roughly five days before ovulation until about one day after, prompting advice to have vaginal intercourse every one to two days, or every two to three. Some couples practicing timed intercourse use urine tests that predict ovulation, which may improve pregnancy and live-birth rates for those who have tried for under twelve months and are under forty. The medical evidence is less clear on whether timed intercourse raises ultrasound-confirmed pregnancies, or whether it changes a person's stress or quality of life. Sex position makes no significant difference to pregnancy rates, so long as ejaculation reaches the vagina. When a sperm donor has intercourse with a woman solely to impregnate her, this is called natural insemination, as opposed to the artificial kind. Reproductive options extend beyond heterosexual pairs. Gay male couples may turn to surrogate pregnancy, lesbian couples to donor insemination or surrogacy, and some women use artificial insemination to become single mothers by choice.

• 5. Protection And Its Limits

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  Correct and consistent use of latex condoms cuts the risk of HIV transmission by roughly 85 to 99 percent compared with unprotected sex, according to reports from the National Institutes of Health and the World Health Organization. Yet condoms are rarely used for oral sex, and far less research exists on condom use for anal and oral sex than for vaginal. Abstaining from vaginal, anal, and oral intercourse remains the most effective way to avoid sexually transmitted infections. Birth-control choices bend to culture, religion, gender roles, and folklore. In the predominantly Catholic nations of Ireland, Italy, and the Philippines, fertility awareness and the rhythm method are emphasized while other methods draw disapproval. Worldwide, sterilization is the more common method, and the intrauterine device is the most common and effective reversible one. The stakes are starkest in developing countries, where one in three women gives birth before age twenty, and where 90 percent of unsafe abortions could be prevented by effective contraception. The 2010 National Survey of Sexual Health and Behavior found that one in four acts of vaginal intercourse in the United States is condom-protected, rising to one in three among singles. That same survey reported that adults using a condom rated the experience just as positively for arousal, pleasure, and orgasm as those who went without.

• 6. The Risk Ledger

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  Nineteen million new cases of sexually transmitted infections appear in the United States every year, and the World Health Organization estimated in 2005 that 448 million people aged 15 to 49 were infected annually with curable infections such as syphilis, gonorrhea, and chlamydia. Many infections cause no early symptoms, which raises the chance of passing them on unknowingly. Some cause genital ulcers, and even those that do not can raise the risk of acquiring or transmitting HIV up to tenfold. The toll concentrates in one region. Of the 2.7 million new HIV infections estimated worldwide in 2010, 1.9 million, or 70 percent, were in Africa, and the roughly 1.2 million Africans who died of HIV-related illness that year made up 69 percent of the global total of 1.8 million deaths. Pregnancy carries its own arithmetic of the unplanned. The World Health Organization estimated in 2005 that 123 million women become pregnant each year, around 87 million of those pregnancies, or 70.7 percent, unintended. In the United States the teenage pregnancy rate fell 27 percent between 1990 and 2000, from 116.3 per 1,000 girls aged 15 to 19 down to 84.5. Sexual activity even leaves a trace in the brain. It can raise expression of the transcription factor delta FosB in the reward center, and frequent daily activity can overexpress it and induce an addiction, sometimes called hypersexuality. One rare form is Kleine-Levin syndrome, marked by hypersomnia and hypersexuality together. Rarer still is coital death, a death triggered by coronary complications during sex, with risk slightly higher for people who get little physical exercise.

• 7. When Bodies Disagree

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  A 2008 survey of Canadian and American sex therapists put the average length of heterosexual intercourse at seven minutes, judging one to two minutes too short, three to seven adequate, seven to thirteen desirable, and ten to thirty too long. The Kinsey Institute had complicated the question years earlier. In 1991 its scholars noted that the time between penetration and ejaculation varies from man to man and from one occasion to the next, and that Kinsey found 75 percent of men ejaculated within two minutes, without ever asking whether the men or their partners found two minutes satisfying. Difficulty rather than duration defines many disorders. Anorgasmia, regular difficulty reaching orgasm despite ample stimulation, is far more common in women than men, with about 25 percent of women reporting difficulty, 10 percent never having had an orgasm, and 40 to 50 percent reporting sexual dissatisfaction or trouble becoming aroused at some point. Vaginismus, an involuntary tensing of the pelvic floor, can make penetration painful or impossible, sometimes treated with vaginal dilators or even Botox. Among men, roughly 40 percent report some form of erectile dysfunction at least occasionally, and the American Urological Association estimates premature ejaculation affects 21 percent of men in the United States. Drugs such as Viagra, Cialis, and Levitra address impotence tied to medical conditions, though doctors warn of serious risks including a raised chance of heart attack. The SSRI dapoxetine has been used against premature ejaculation, while delayed ejaculation can arise as a side effect of antidepressants such as fluvoxamine. Intercourse often remains possible after major medical treatment, including extensive gynecological surgery such as hysterectomy or cancer treatment, with reconstructive surgery an option for some women.

• 8. Law, Faith, And The Animal World

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  More than 90 percent of rape victims are female, 99 percent of rapists are male, and only about 5 percent of rapists are strangers to their victims. The legal meaning of the crime has shifted in living memory. Robert Francoeur and colleagues noted that before the 1970s, rape definitions often included only penile-vaginal intercourse, which left forced oral sex and other acts outside the category. In 2012 the Federal Bureau of Investigation replaced its old wording, the carnal knowledge of a female forcibly and against her will, with a far broader definition covering penetration of the vagina or anus by any body part or object, and oral penetration by a sex organ, without consent. Most countries set an age of consent between 16 and 18, with the full range running from 12 to 20. Marriage long carried its own rules. Many religions require consummation, and a failure to consummate can be grounds for annulment, which needs no divorce. Until the late 20th century a marital exemption in rape laws usually shielded a husband from prosecution for forced sex with his wife. Faiths diverge sharply on the act itself. The Shakers held that sexual intercourse is the root of all sin and that all people, even married couples, should be celibate, and their community, which peaked at 6,000 members in 1840, had dwindled to three by 2009. Judaism obliges a husband to provide his wife sexual pleasure called onah, written into the marriage contract, the ketubah. The patterns extend well past humans. Bonobos, chimpanzees, and dolphins are known to have sex even when the female is not in estrus, and to engage in same-sex behavior, signs that intercourse in these species has come to serve social bonds beyond reproduction.