Make no mistake: Women can be just as driven as men are in pursuit of a fling. One woman Smith met used cheating as a weight-loss incentive, telling herself that a certain guy would sleep with her if she dropped a particular number of pounds. Once she succeeded, she was on to the next quarry. Another one waited for her husband’s Ambien to kick in so she could go across town for the night and be back by the time he awoke.

“Women have become, in many ways, as predatory as men,” says Judith Brandt, the author of The 50-Mile Rule: Your Guide to Infidelity and Extramarital Etiquette. And the prey is abundant. We grew up with the bejesus scared out of us by Anjelica Huston in Crimes and Misdemeanors and Glenn Close in Fatal Attraction. The libido-withering moral was clear: It’s just not worth it, man. But where’s the male equivalent? Your wife’s potential playmate probably has no interest in annexing your emotional territory.

And he’s accessible: Today’s wife knows nothing of the isolation of her mid-century counterpart. She has Internet chat rooms and cell phones. She has personal trainers, yoga instructors, and mommy groups. Mommy groups? Why would any man think he had reason to get twitchy about his wife’s going to a mommy group? Jerry, a 32-year-old engineer from Atlanta, certainly didn’t. After the birth of his son, his stay-at-home wife joined one and met a woman who soon became her best friend. She’d go to her new friend’s house for barbecues, and occasionally she’d end up sleeping over in the spare bedroom. After several months of this, Jerry began to suspect that his wife was being unfaithful. He started snooping—looking at her cell phone when she wasn’t around and using Google Desktop to comb for damning e-mails. One day he found a message referring to a certain lapse in judgment that occurred on the hallway floor with the husband of his wife’s girlfriend. When he confronted his wife, she told him when it had all started: six months before, on the day after Father’s Day. They were staying with his parents because they’d put their home on the market. Jerry had the day off. His wife told him to relax with their son while she went to clean up their house for prospective buyers—a tidy little cover, it turned out, for a secret meeting that had nothing to do with real estate. “She said she was shampooing the rugs,” Jerry says. “I never suspected anything. I mean, she did shampoo the rugs. My attitude was, she’s my wife—I’m supposed to trust her.”

Like Rob, Jerry opted to stay in the marriage. According to studies conducted by Frank Pittman, a psychiatrist and the author of Private Lies, more than a third of marriages in which infidelity occurs end up surviving. “It doesn’t make you stop loving her,” Jerry says. But it could inspire you to install spyware on your computer and go to marriage counseling—two measures Jerry took. “In Georgia she’d get half my salary, and she’d probably get custody of the child,” he says. “I would probably lose my house, and I lose my son—and I didn’t do a damn thing to cause it.”