More women have affairs than men do.
If the true statistics were know, you would see that more women have screwed around on their partner than men have.
It is a know fact that women are desired by men. Matter a fact, it's much easier to get laid as a woman than as a man. You can see than in pretty much every species on the planet. So if that were true, and it is. Then logic dictates that for a woman to have an affair, she only need to say yes to the next man flirting with her. Whereas men have to convince a woman to have sex with him, a difficult task which is compound if he is already committed to another partner.
So. If it's harder for men to have sex with women being single, and then compounded if he's in a relationship already, then then his odds of having an affair are much much lower than a woman who only needs to say yes to the next guy who propositions her.
Ergo, logic dictates that more women have had affairs than guys, because it's so much easier for them to screw around.