How to cheat-proof your love: If you do decide to live together, Popenoe says, don't use it as a trial for marriage. That mindset "is almost the opposite of the one you need to have if you want a long-term commitment: Let's see if this works, and if it doesn't we can get out easily." If your guy's got a history of serial monogamy, the risk escalates: "After a whole series of relationships like that, it's very difficult to jump into one where you've got to work everything out and can't run away," Popenoe says. And if a guy feels trapped, an affair can seem like an easy way out.
The Web makes cheating easy.
It's simpler than ever to find an affair online -- and the people searching for one are not always those you'd suspect. Of the 3.3 million users of AshleyMadison.com, a dating site that unapologetically caters to married people looking to stray, about 500,000 are newlyweds, according to the site's president, Noel Biderman.
Even if a person doesn't go online specifically to troll for a dalliance, the very nature of the Internet can be the start of a slippery slope toward infidelity. First, there's the easy access to pornography: The University of Washington data found that men under age 35 were two and a quarter times more likely to have cheated if they had seen an X-rated movie. "Consumption of pornography might lead to increases in specific types of sexual behaviors, including anonymous sex," says James Furrow, Ph.D., a co-author of the study and the Freed chair of marital and family therapy at the Fuller Theological Seminary School of Psychology.
Then there are sites like Facebook, where anyone from exes to one-night stands can find you. What starts as innocent e-flirting (which 20 percent of adults who used social-networking sites in 2008 admitted to in a study by the Pew Internet and American Life Project) can quickly get out of hand.
Mark, 28, a financial analyst from Connecticut, began using online dating sites when he was single and hasn't stopped, even though he got married in 2004. At first he just chatted online, but eventually he started arranging dates in cities he traveled to for work. He had his first fling within 16 months of his wedding and has racked up four more since then -- none with anyone from his hometown. "The feeling of having e-mails from women across the country in your inbox is exciting," he admits.
How to cheat-proof your love: Not every guy with an e-mail account is going to stray, and checking your man's browser history will only show you don't trust him. It's important to remember that most men who use the Internet to have an affair are looking for sex, not intimacy, says Mary Jo Rapini, a psychotherapist at Methodist Hospital in Houston who specializes in sex and intimacy issues. Given that, Rapini advises circumventing the temptation by watching a steamy flick or even some light porn with your hubby. "You're taking away the sneaking-around element and instead enjoying it together," she says. "And that usually ends up turning you both on."
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