[It is no surprise to an evolutionary biologist] that the act of love...should be fraught with conflict. Sex is the most concentrated source of physical pleasure granted by our nervous system, so why is it such an emotional bramble bush? In all societies, sex is at least somewhat “dirty.” It is conducted in private, pondered obsessively, regulated by custom and taboo, the subject of gossip and teasing, and a trigger for jealous rage....

...The reasons are as deep as anything in biology. One of the hazards of sex is a baby, and a baby is not just any seven-pound object but, from an evolutionary point of view, our reason for being. Every time a woman has sex with a man she is taking a chance at sentencing herself to years of motherhood, with the additional gamble that the whims of her partner could make it single motherhood. She is committing a chunk of her finite reproductive output to the genes and intentions of that man, forgoing the opportunity to use it with some other man who may have better endowments of either or both. The man, for his part, may be either implicitly committing his sweat and toil to the incipient child or deceiving his partner about such intentions.

And that covers only the immediate participants. As [novelist Erica] Jong [has] lamented..., there are never just two people in bed. They are always accompanied in their minds by parents, former lovers, and real and imagined rivals. In other words, third parties have an interest in the possible outcome of a sexual liaison. The romantic rivals of the man or woman, who are being cuckolded or rendered celibate or bereft by their act of love, have reasons to want to be in their places. The interests of third parties help us understand why sex is almost universally conducted in private. [Anthropologist Donald] Symons points out that because a man’s reproductive success is strictly limited by his access to women, in the minds of men sex is always a rare commodity. People may have sex in private for the same reason that people during a famine eat in private: to avoid inciting dangerous envy.

As if the bed weren’t crowded enough, every child of a man and a woman is also the grandchild of two other men and two other women. Parents take an interest in their children’s reproduction because in the long run it is their reproduction too. Worse, the preciousness of female reproductive capacity makes it a valuable resource for the men who control her in traditional patriarchal societies, namely her father and brothers. They can trade a daughter or sister for additional wives or resources for themselves, and thus they have an interest in protecting their investment by keeping her from becoming pregnant by men other than the ones they want to sell her to. It is not just the husband or boyfriend who takes a proprietary interest in a woman’s sexual activity, then, but also her father and brothers. Westerners were horrified by the treatment of women under the regime of the Taliban in Afghanistan.... [Evolutionary psychologists Margo] Wilson and [Martin] Daly have shown that laws and customs with the same intent—giving men control over their wives’ and daughters’ sexuality—have been common throughout history and in many societies, including our own. Many a father of a teenage girl has had the fleeting thought that the burqa is not such a bad idea after all.

On strictly rational grounds, the volatility of sex is a paradox, because in an era with contraception and women’s rights these archaic entanglements should have no claim on our feelings. We should be ziplessly loving the one we’re with, and sex should inspire no more gossip, music, fiction, raunchy humor, or strong emotions than eating or talking does. The fact that people are tormented by the Darwinian economics of babies they are no longer having is testimony to the long reach of human nature.