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Last year, researchers at the University of Washington Center for the Study of Health and Risk Behaviors who analyzed data on infidelity taken from the General Social Survey found that roughly 20 percent of men and 15 percent of women under age 35 copped to cheating on their spouses in 2006 (the latest figures available), up from 15 and 12 percent, respectively, 15 years earlier.

What gives? Part of the problem, says David Popenoe, founder and co-director of the National Marriage Project at Rutgers University, is that we've become an 'anything-goes' society, in which the indiscretions of politicians, sports figures, and pop celebrities are practically daily news.

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Also, "newlyweds have this sense about cheating that if you're going to do it, do it now," says M. Gary Neuman, an infidelity expert and author of The Truth About Cheating. "Men may feel that if the marriage was a mistake, it's better to figure it out before things get even more entangled with kids and families." And no doubt while their wives are least likely to be suspicious. But there are ways to protect your young marriage. The key is to be proactive—don't assume that uttering "I do" ensures fidelity.

Here are the top reasons why newlywed men cheat, and how you can take action to make sure your guy doesn't.

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1. You've played house for years

Time was, being a newlywed meant finally getting to share a roof. Not so today. More couples than ever are shacking up—6.4 million in 2007, compared with fewer than 1 million 30 years ago, according to the Census Bureau. Add the time you've lived together to the average 17-month engagement, and it's a good bet the attraction is less electric by the time you walk down the aisle.

Research shows that infidelity rates are much higher among cohabiting couples than married folks who don't live together first. One possible reason: "Often, a couple that decides to live together isn't as committed," Popenoe says. And if that's the attitude, he adds, it doesn't necessarily change just because you get married. The possible result: In fairly short order, one of you—most likely him—is on the prowl.

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.

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2. 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, 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."

3. Marriage hits guys harder

Compared with dating and an engagement, marriage is serious business. It can seem like a drag, especially to men. "They feel this sudden responsibility to be a good provider and a good husband," says Terri Orbuch, a marriage therapist and research professor at the University of Michigan. "These expectations can make young men feel old and boring."

Ruth Houston, author of Is He Cheating on You? 829 Telltale Signs and founder of infidelityadvice.com, has conducted more than 10,000 interviews with cheaters and says: "Men want to recapture how they felt when they were dating, and if they can't do it with their wives, they'll look elsewhere."

How to cheat-proof your love: Since the early stages of marriage can be a vulnerable time for him, get vocal about how much you appreciate him—both as a husband and as the smokin'-hot guy you fell in love with. Thank him when he cleans the bathroom and remind him that he's sexy, smart, and cracks you up. "Re-creating the excitement you once had can actually boost dopamine and oxytocin, two brain chemicals responsible for that love rush," Weil says. So indulge in the occasional spontaneous road trip or lunchtime quickie so that your passion can thrive within the marriage, not outside it.

4. The sex has gotten stale

"It's not necessarily that the frequency of sex declines in that first year, but some of the passion dies down," Orbuch says.

That was the case for Doug, a 33-year-old teacher in Texas who dated his wife for two years before marrying her. When their sex life ran dry a little over a year into the marriage, he started sleeping with an ex-girlfriend. "I felt like I needed it," he says.

Part of the problem in cases like Doug's: "While all the companionship and familiarity of marriage makes a couple closer, it can kill the fire in the bedroom," says Patti Britton,  author of The Art of Sex Coaching. A few years ago a German study found that women's libidos steadily decreased while they were in a secure relationship, while men's drives stayed the same.

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How to cheat-proof your love: Talk it out, Weil advises. "Make sex a priority and resolve to have it at least a few times a week." Consider coming up with your own personal "sex vows": I promise not to have a headache for more than three consecutive nights. I promise I'll be open to trying new things in bed, and so on.

That doesn't mean you have to tackle the Kama Sutra every time you knock boots. In studying men who cheat, Neuman found that those who were dissatisfied with their marital sex lives preferred quantity over quality. "A man would rather have basic sex a few times a week than swinging-from-the-chandeliers sex every once in a while," he says. That's not to say you should shelve the acrobatics and lingerie—just agree to get it on more often, with or without props.

5. Marriage didn't fix him

You'd think that commitment-phobes would avoid the altar, but often they go through with marriage thinking that it will "cure" them. When it doesn't, they feel stuck.

"People who have cheated in the past usually can't handle being so close with one person. But that's exactly what happens in that first year of marriage—you get closer, you plan for the future, you focus on the togetherness and the partnership," Weil says. "So someone with commitment issues might feel even more trapped, and be more compelled to cheat, using another person as an intimacy blocker to keep themselves from getting too close to their spouse."

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John, the pharmaceutical rep who cheated after having been married for a year, says he had always cheated on his girlfriends. "I honestly think it's just the way I'm wired, like a lot of guys."

How to cheat-proof your love: Your best bet is not marrying a guy who's at high risk for straying. Houston offers these warning signs: an extremely active sex life, a lot of female friends, a lot of male friends who cheat, and parents (a mother or father) who cheated.

*Names and details have been changed to protect the sources' anonymity.