Spitzer, Edwards, Sanford, Letterman. Not since Clinton rolled a Cuban in Lewinsky has our country so gorged itself on the scurrilous details of extramarital dalliances. Thousand-dollar hookers. The predilection for doing it "raw." The secret love child. The back entrances at the Beverly Hills Hilton. Those e-mails about "magnificently gentle kisses" and "tan lines." It's as if some evil scientist had activated a microchip in all of us that made us behave like goats. One click, one maniacal cackle, and Gomorrah is upon us.

As a nation, we did our part in each instance by exhibiting the requisite outrage and disgust. We devoted airtime and newsprint to lengthy discussions about the libido of the powerful male, his insatiable appetite and subconscious propensity for self-destruction. We wanted answers. We wanted justice. Most of all, we wanted to believe that this was the exception and not the rule—when, in fact, everyone from the club-prowling playboy to the Similac-smeared Dad of the Year is prone to—likely even wired for—this behavior.

What none of us want to consider when we get to that "forsaking all others" clause in our marriage vows is that infidelity is more common than obesity in this country. According to a recent University of Washington study, 28 percent of men will cheat on their wives at some point in their lives. By comparison, only 25 percent of Americans qualify as fat, according to a 2008 report by the Centers for Disease Control. And when you lower the stakes, adultery seems to become even more attractive: 74 percent of men say they'd have an affair if they knew they'd never get caught, reports InfidelityFacts.com. Somewhere between "I do" and "Be sure to leave the light on," we became the men we said we would never be—the kind who kiss their wives good night and then fantasize about the redhead who was on the next elliptical that morning. We've spawned a cottage industry with our bad behavior: from private investigators and reality-TV shows dedicated to nailing the cheaters to AA-style support groups, weekend retreats, and crisis centers committed to healing the victims.

"A lot of people are coming to terms with the unnaturalness of monogamy," says David P. Barash, coauthor of Strange Bedfellows: The Surprising Connection Between Evolution, Sex and Monogamy. "But there's a difference between the public persona—what we like to think of each other—and what we all know goes on." Barash, a zoologist and psychologist, has spent years debunking the notion that we have it within ourselves to remain faithful for long stretches of time. Turns out it's just as unnatural for man as it is for almost any other member of the animal kingdom. One notable exception is Diplozoon paradoxum, a tiny parasitic worm that inhabits the intestines of fish and mates for life—but really, what are your options in there?