While it is amusing to come up with "reasons" people venture into infidelity, it may be more useful to examine the fundamental premise: that monogamy is a natural state for humans. In fact, both anatomy and behavior show clearly we have the worst of all worlds: we are a semi-monogamous species. Thus it is in fact almost impossible to be content with long-term monogamy unless one has very limited expectations and desires. We ought to stop babbling about "cheating" as though life were a children's game with simple rules, and instead look at the underlying reality of the situation. When we do that, we can see our social norms are hopelessly at variance with our fundamental needs.