Sexual infidelity is one of humanity’s great obsessions, perhaps second only to violence. We abhor it, yet we want to hear all about it. We love the topic. It’s on the news, the web, movies, and on TV. We can’t get enough of it. But while we often get the details of “who” the infidelity involves and “how” it was carried off, the question of “why” is the most vexing.