When the fight for gay marriage began to gain traction back in the early years of the last decade, social conservative critics usually went beyond denying that marriage could be redefined to include same-sex couples. Many of them argued that homosexuals were much less inclined than heterosexuals to valorize the ideal of monogamy. Allowing gays and lesbians to marry would therefore introduce a polyamorous option into the institution, and adultery would come to be viewed as an acceptable option for all marriages.

A spate of recent articles explicitly making the case for polyamory would seem to vindicate those conservative predictions and worries.

The latest example appeared a few days ago in The New Republic (reprinted from the New Statesman). Three and a half years into a relationship with a woman, author Rosie Wilby finds that "other people act as our kindling. Love breeds love." The experience leads her to ask rhetorically: What if we all came to view "our relationships as a pyramid structure with our primary partner at the top and a host of lovers, friends, spiritual soul mates, colleagues, and acquaintances beneath that?"

That's the polyamorist ideal, which Wilby defines as "consensual multiple loving connections, some sexual, some not, in a myriad of combinations and hierarchies." And, she insists, it's far better for everyone involved (including children) than the serial monogamy that she sees practiced by many straight and gay couples alike, leading to high rates of separation, divorce, and broken homes. "Instead of serial relationships one after the other," perhaps it would be better to foster "parallel ones running alongside one another." And anyway, we already act this way, fantasizing about sex with friends and acquaintances, making emotional connections with people other than our spouses or partners, sometimes cheating on them outright. So "why pretend" we're more monogamous than we really are?

A social conservative would probably note, correctly, that Wilby's essay is perfectly congruent with, and even seems to follow of necessity from, the sexual ethic currently sweeping the Western world — one in which the only valid moral consideration in a sexual relationship is individual consent. In such a moral universe, there is no reason not to embrace a polyamorous lifestyle. And since most human beings will find themselves emotionally and physically attracted to multiple people in the course of their lives, monogamy would seem to be doomed.

There's just one problem: There is not one shred of evidence to support that prediction.

Consider: In a poll conducted just last year, Gallup found that 91 percent of Americans disapprove of marital infidelity.

That's right. In a highly sexualized age awash in technological temptations and dominated by a nonjudgmental sexual ethic that increasingly encourages men and women to do whatever feels good, nine out of 10 Americans judge cheating to be wrong. That's higher than the rate of disapproval for human cloning and suicide.

What to make of the disjunct between what our principles would seem to permit or encourage and what we clearly believe to be right and wrong?

One possibility is that people's attitudes haven't caught up to the implications of their moral ideals. Once they do, the rate of disapproval will fall far and fast.

I suppose it could happen. But since there's currently no evidence for it — not even a modest downward trend — buying into the theory would seem to be ill-advised.

That's why I prefer another explanation — not of why Americans disapprove so strongly of adultery, but of why advocates of polyamory, no less than their social conservative antagonists, get the trend lines wrong.

Those who promote polyamory often end up making a very basic error in moral reasoning by presuming that one can extrapolate moral ideals from people's behavior. If people cheat and are increasingly tempted to cheat, their professed attachment to the principle of monogamy must be (as Wilby puts it) "pretend."

But the tradition of moral reasoning that comes down to us from Judeo-Christian civilization makes a very different moral presumption — namely, that due to sin, temptation, and weakness of will, human beings will often, even usually, fail to live up to the moral ideals they profess. The fact of that failure doesn't make the ideal any less real or binding, just as the prevalence of murder does not lead us to doubt it is an act of evil. An ideal remains an ideal, even when we fall short of it.

I can't say precisely why Americans believe so strongly in the ideal of monogamy. But they do, and the mere presence of temptations and an attitude of permissiveness in our culture shouldn't be taken as evidence (by either the advocates or opponents of sexual liberation) that they're on the cusp of rejecting that ideal.

On the contrary, I'd say that America's peculiar mix of unfettered sexuality and stern disapproval is much more likely to produce a culture marked by moral confusion, anxiety, and self-loathing. It certainly won't be the free-loving, happy-go-lucky, hippie-commune culture that the polyamorists crave and the social conservatives dread.