It's boring to ask why men have affairs, so let's just talk about why women have affairs.

Emily Bazelon wants to talk about infidelity:

Like everything else about male sexuality, the male desire to lie with another woman is boringly uncomplicated. But why do women have affairs? The judgment of literature (Anna Karenina, Madam Bovary) is that they feel trapped and oppressed, or, less sympathetically, that they're easily gulled by preying males one or two notches up the social ladder. Two centuries later, I would imagine that life is a bit different. The answer we heard from writers like Erica Jong and Gael Greene back in the swingin' Plato's Retreat 1970s was that women crave sexual variety in precisely the same way men do. Three decades later, though, feminism no longer insists that women's desires and inclinations be identical to those of men. It may even be permitted to recognize that, at least superficially, the female sex drive seems, in the aggregate, less pronounced (or at least less conspicuous) than the male sex drive. You don't hear stories about men telling their wives they no longer want to have sex. You do hear stories about women telling their husbands they no longer want to have sex.

So, if sex, especially variety in sex, is less important to women, we must have more complex and fascinating reasons for committing adultery.