“Nonmonogamy” is a gentle mouthful of a word, a polite replacement of sorts for “promiscuity” and “infidelity” in the lexicons of cosmopolitans. The same kind of transformation has happened to “virginity loss,” “cheating,” and “prostitute,” terms no longer considered appropriate for the more sex-positive among us. In their place we now have sexual debut, extradyadic sex, and sex worker, respectively. As a student of young Americans’ sexual behavior, I’ve found that the lingo alone requires effort to master. But neither my word processor nor even most of my hipster neighbors recognize the legitimacy of nonmonogamy—the practice of supplementing a primary sexual partner with one or more others.

Serial monogamy, however, is another story. In fact, it’s the primary sexual script among young adults today. And it’s into this pattern that most Americans of any age put their energy. You’re only allowed one sexual partner at a time, and to overlap is to cheat, and cheating remains a serious norm violation that gives the victimized party not just the uncontested right but often a perceived moral obligation to end the relationship. And that, argues the authors of Sex at Dawn, is a problem.

Apparently we didn’t just evolve from apes. We are apes. When it comes to matters of sex, authors Christopher Ryan and Cacilda Jethá charge, we haven’t really even evolved. Although we more closely mimic chimpanzees, who fight and feud over sex, we would do well—the authors assert—to mimic the Bonobo, that amiable chimp cousin who appears both gracious and generous in its sexual expressions. Bonobos resolve their power issues with sex. And their anxiety issues. And pretty much any issue. Make love, not war, is their mantra.

Monogamy, Ryan and Jethá claim, is not natural. The nuclear family doesn’t become us. The two spend much of the book’s 312 pages amassing evidence for their case against monogamy past and present. Much of it comes from looking at bodies and practices today, deducing why such things exist in the shapes and lengths that they do, and what that must’ve meant about human social and sexual behavior many millennia ago. They call it the “hieroglyphics” of the human body. Anything that’s still with us—like the comparatively large penis and testicles—must be the way they are for some evolutionarily expedient reason.

Sex without exclusivity, the authors assert, is one such example. It was our ancestors’ modus operandi before they decided to give farming a try. When we foraged, we not only shared our food with each other, we shared our bodies, too. Whereas modern man wishes to know that he is the biological father to his children, Ryan and Jethá claim that our foraging fathers would neither know nor care. He spread his seed—and she her legs—for the public good. Sperm competition, rather than power dynamics within communities, decided who’s your daddy.

As a sociologist, I don’t know enough about anthropological methods, physiological realities, or about the study of evolutionary eras, to know whether Ryan and Jethá have gotten their assessments about our great-great-great ancestors right or not. Nor will most readers. So I will give them the benefit of the doubt. Their account of blissful forager life may well be true. But just because young Mosuo women in remote China give it up to whomever they wish doesn’t mean that life would be swell if we would.

But, the authors claim, we do give it up, and they’re half-right about that. Not all of us, of course, not by a long shot. But even the chaste among us spend time resisting this basic drive. And that, the authors urge, is evidence that monogamy is not natural. I confess I’ve never spent much time on this question. Is monogamy natural? I have no idea. Perhaps it isn’t. I do, however, think monogamy is good. It’s functional, even. And it’s hard. And it remains the aspiration, if not the actuality, of the majority of Americans (and not just the religious ones).

Most of us—especially women—are invested in monogamy. I realize it’s politically unpalatable to assert it, but most women just aren’t interested in having sex with lots of different men. I know, I know—there are some seriously horny women out there. But they’re not the norm. Social psychologist Roy Baumeister reminds us that on nearly every measure and every dataset, men want sex more. They masturbate more (which is probably the purest measure of sex drive differences). They initiate sex more often and refuse it less often. They take more risks for sex. (Every few weeks or so this one is made obvious anew in the media.)

The social control of sexuality is a trade-off, Ryan and Jethá claim, one that commenced with the agricultural era. Life was better when we just foraged, they surmise. (Seriously?) While the distant past is beyond my speculative abilities, present-day social control of sexuality remains necessary, even in a freedom-loving and rights-obsessed democracy like ours. Why? Because surrendering to desire can and does contribute to no shortage of personal and public misery.

But trying to rise above our nature is equally challenging. It’s an “exhausting endeavor,” Ryan and Jethá caution, “often resulting in spectacular collapse.” To be sure, but to believe that loosening sexual standards like monogamy means that everyone will be free to do as they please is simply foolish. Human communities don’t work that way. If a critical mass ever successfully snubs relationship commitment, permanence, and sexual exclusivity—and that is a possibility, given ample time—it will become remarkably difficult for a minority to do otherwise. There will always be rules—with resulting winners and losers—in any sexual system. Ryan and Jethá would like some new—or rather, some very old—rules.

And they see evidence around us of exactly that. I agree that the times are changing. “In terms of sexuality,” Ryan and Jethá conclude, “history appears to be flowing back toward a hunter-gatherer casualness.” I wish it weren’t so. But in my national study of 18-23-year-olds, 36 percent of men’s relationships became sexual in less than two weeks. Three in ten relationships don’t even have a romantic component to them. No wooing, no dates. And even though most young adults still pay deferential lip service to marriage, they’re voting with their feet: among 25-34-year-olds, the share of them that are married has declined another 10 percent since 2000. Cohabitation, on the other hand, jumped 13 percent in 2010 alone. Something is afoot.

That something, Ryan and Jethá proclaim, is a return to our nonmonogamous past, the one they surmise our bodies were destined for. The sociologist in me, however, prefers answers rooted in more recent—if less exciting—developments. Indeed, my favorite answer for explaining today’s sex scene is a simple one at face value: it’s because women no longer need men. Like them? Yes. Need them? No.

Back when women needed men, women protected and policed each other in the domain of relationships. Men, in turn, saw one valid option for accessing stable sex: marriage. That is no longer the case, assisted in no small part by economic changes and reproductive technologies like the Pill, whose 50th anniversary was marked last year. Not only does birth control work as advertised—enabling women to control their fertility—it also functions to weaken the connection between sex and strong commitments. It really can’t be otherwise. It’s a trade-off Westerners have elected to live with.

“Societies in which women have lots of autonomy and authority,” the authors deduce, “tend to be decidedly male-friendly, relaxed, tolerant, and plenty sexy.” It’s true. But then try getting men to do anything. Sure, it’s not the only contributor to men’s newfound failure to thrive, but it is where American life is trending today, as Hanna Rosin hints in her Atlantic Monthly article entitled “The End of Men.” Even Ryan and Jethá recognize this, noting Sigmund Freud’s observation that “civilization is built largely on erotic energy that has been blocked, concentrated, accumulated, and redirected.” Roger that. If monogamy was the price of civilization, I’m glad our “Founding Farmers” paid it.