Wherever you turn everyone seems to be happy to talk about sex, and even to have some. But there is a kind of sex you are not allowed to talk about, let alone admit you are having. Even though it is legal. Even though research shows that it is very common, especially among young people (a national survey in the U.S. published this year found that 40% of men and 35% of women between ages 25 and 44 have tried it, an increase of almost 50% from the 90s). Even though it is mentioned in the Bible, portrayed in fine literature, and depicted in ancient art. Even though it is starring on the Internet. Even though it raises fascinating questions about the nature of human sexuality.

Thought experiment: When was the last time you talked to your friends honestly, openly--deeply--about anal sex? Many readers may respond with a reflexive ‘ew,' or the obligatory ‘yuk.' But that response is worth reflecting on. Why the recoil?

Some say it's not natural. This argument is weak on its face. Human nature includes the full range of human expression, from altruism to cannibalism. In addition, the need to subvert, challenge, and contradict human nature is one of the defining characteristics of the human race. In a sense, flying is not natural. Neither is walking on high heels; or eating with a knife and fork. Unnatural behavior is part of our nature.

And what is natural sex? All the sex surveys show that the central sexual norm is subjectivity. "A nymphomaniac," said sex researcher Alfred Kinsey, "is someone who loves sex more than you." Some would say that sex is natural when our anatomical parts are used for their biological purpose. The rectum is not intended for penile penetration. But neither is the mouth, and yet oral sex does not now have the same stigma. Anatomically, your headache is not intended to signal that you're missing Ibiprophen in your body. Yet you reach for the medicine cabinet. The night does not fall in order that you may turn on the lights, and yet you do, and no one calls this "unnatural."

If the biological purpose of sex is to have children, then we deny our nature continuously, vigorously and thoroughly. Most sex that is going on right now around the world is not for procreation. Researcher David Buss and his team have found that women, for example, list 237 other reasons to have sex; some are obvious (love, pleasure, horniness), others less so (boredom, revenge, insurance against the departure of a boyfriend). Other reasons included the frankly manipulative (make him take out the garbage), health concerns (treat a headache), spirituality (get closer to God, experience enlightenment), or altruistic impulses (give pleasure to another, boost his self-esteem.)

Some say it's dirty. Yes, the anal sex scene is not sterile. But we have a complex relationship with the things we define as 'dirty'. We do not only hate dirt. We are also attracted to it. Especially if we think there's something valuable buried in it. People will happily mine dirt to find gold. Playing in 'dirt' in itself can inspire delight; think of kids playing in the mud, and adults gossiping. Dirt can also be a symbol of status. The worker wears the dirt on his hands with pride to show his effort, commitment and strength. Not everything we take as dirty (menstruation, in certain traditions) really is. And what is actually dirty (cigarette smoke, for example) we often ingest happily, feeling cool.

Some say it's not socially acceptable. There are people who manage their sex life like lousy politicians decide policy: according to the latest polls. Sex with such people tends to be as dreary and depressing as the policies of such politicians. The authentic sexual encounter allows, even demands, the creation of an independent territory; a private world where our own private rules apply and no others. Human sexuality is not a fish pond but an ocean, complete with deep streams, secrets, and the struggle for survival. In good sex we are fully alive, and the world be damned.

Some say it hurts. True, it can hurt, if you approach the act without preparation, lightly, in ignorance, stress, or haste. But a lot of things in life are painful, and still we talk about them and do them. Pregnancy and childbirth. Military service. Love. Relationships. Marriage. Divorce. Marathons. Tattoos. Taxes. Pain is not a bug in our software but a feature of our hardware. Those who cannot suffer any pain, particularly short-term pain, doom themselves, ironically, to a painful existence.

Some say it's dangerous. Yes, anal sex involves taking risks. One risk is that you'll have to admit to yourself you're doing it. But the main risk is an STD. Anal sex without a condom is the most dangerous sex act in terms of your health. According to the U.S. Center for Diseases Control, the chance of getting AIDS through oral sex is 1 in 10,000 sex acts. The chance in vaginal sex is 10 in 10,000. With anal Sex: 50 in 10,000. But the fact that anal sex is dangerous is actually a good reason to talk about it honestly. Suppressing discussion and promoting ignorance are not useful ways to deal with danger.

In addition, the truth is that life has risks. In fact, life is a risk. You die from it. The question in life is not: Are there risks? The question is: Is it worthwhile for me to take that risk? And: How can I reduce the existing risk? (Although for many the risk itself is the draw). Is anal sex worth the risk? That, of course, is a subjective question. Some may deem the risk of disease, pain, embarrassment, and failure too high. But others may say, for whatever reason, that the risk is worth taking.

At this point, my three remaining loyal readers might feel tempted to ask: Why do people--and women, who incur most of the risk, in particular--do this? Reliable scientific data are missing here, and what people say about their motives does not necessarily reflect the motives themselves. But here are a few things we think we know:

First, some women enjoy it. The author Toni Bentley, in her strange and compelling book Surrender from 2004, describes the experience as a deeper, more meaningful penetration, "on the edge of sanity" ... a way to experience eternity in one moment, a spiritual, divine revelation.

Some women enjoy breaking the taboo. Others are looking to refresh or challenge their sex lives, explore the boundaries. In addition, some may do it to impress or excite a partner.

Paradoxically, although the act is perceived as low and dirty, many couples may perceive it as involving a higher level of intimacy than conventional sex, since it requires high levels of mutual trust, close cooperation, and openness.

Current studies also show that women engaging in anal sex have more orgasms. Here it is important to clarify that correlation does not imply causation. In fact, the correlation between orgasm and anal sex is not easy to account for causally. Some women reach orgasm through anal sex. Some may not agree to anal sex until they have experienced an orgasm. Some women's self confidence may cause them to both be more orgasmic and more open to sexual experimentation. It is also possible that orgasmic women are more open to sexual experimentation because sex in general is more fun for them. In summary, as Woody Allen said: "Love is the answer. But until we find it, sex raises some very interesting questions."

In addition, it is clear that we cannot ignore the element of aggression embodied in the act of sex in general, and anal sex in particular. Heterosexual sex involves an aggressive component. Testosterone, the aggression hormone, is also a hormone of sexual arousal, in both sexes. To the little child who catches her parents in bed, they would seem to be wrestling, crying and wailing, with painful expressions on their faces. Anal sex may express a variation on the aggression theme.