Meh. Both sexes do this. It's pretty sad, too. There's a good chunk of people out there who find married people more attractive and much "safer" to chase after and hook up with than singles. Being married means you've been (hopefully) thoroughly "vetted" by your spouse and deemed trustworthy and worthy of marriage. A married man with children has proven that he's fertile, able to take care of a woman, successfully reproduce with her and will stay with her to take care of their children. A married woman with children has proven that she's fertile, able to please a man and keep him happy enough to knock her up and stay with her, and so on.
Married people are also viewed as "safer" by the homewrecker crowd. They're much less likely to carry any STDs. Married women are much more likely to be on birth control (so raw-dogging is on the table with little to no risk) and even if they do get pregnant they're very likely to lie to the husband and claim it's his to avoid being exposed. Married men can't be as cavalier with their raw-dogging as married women can, but to a woman homewrecker that's an acceptable risk because she knows he's likely to pay for an abortion or "plan B" if there's an accident because he'll want it to stay quiet, and it gives her power over him.
Homewreckers also rely on the fact that married people have a vested interest in remaining "discreet" about their affairs. This saves the homewrecker potential embarrassment if things stay quiet, and gives them a way to blackmail their married lover if they ever want to ("if you don't buy me X I'll tell your wife"). Then there's that whole "thrill of doing something naughty" and "eating the forbidden fruit" thing.
People just love to chase thrills no matter who it hurts.
Modern marriage is fuckin' scary, man. There's an entire mob of people constantly gunning for your spouse specifically because they're married and unless you're a total fuckup like Ryan it's pretty easy for your spouse to cover their tracks.