While cheating can be extremely damaging to most relationships, an affair actually begins long before the act itself. Having an affair is often one person's way of signaling to their partner that something is wrong in the relationship. And, often one person uses cheating as a catalyst to either fix or flee from the problems.
The reality is there are too many unsatisfying and empty relationships. Consequently in today's instant gratification society, at least half of men and women are looking for a quick fix of what they need outside of their relationship. If cheating is easy and there is a high payoff, why wouldn't they?