The easiest, probably most accepted answer behind the reason why men cheat is simple. Men are dogs and that is all there is to it! Okay, fair enough. I guess the better question to pose is, "what are the motivations behind the actions of a man that cheats?" To deeply understand why men are driven to cheat, I encourage you to think about the following presupposition.
Men are biologically driven to procreate.
That is, men are innately imbued with a sense of having sex and procreating. I really think there is simply no way of getting around this idea. Men in fact are hormonal and this hormone is called, "testosterone". It begins at puberty and ends at the grave. This hormone colors our thoughts daily to some degree and thus guides our life to a large degree. It gets us up in the morning for work, it prepares us for war, it motivates us to marry (in part), and it gives us the energy to persevere through all obstacles life. Without testosterone, men simply would not be bothered with half the things we do. Think about the results to an animal when you neuter it, or even the expression of a neutered man. Does it conjure images of strength and vitality?
Having ascribed a biological reason for we men cheat, I'm not endorsing or condoning it. As men we should not do that. Better to stay single if we cannot reconcile those urges with inside of a committed relationship. So how can men rein in those urges enough to stay committed? I think that question is particular for every man, but understanding what makes your man feel like a man is important. It's not to be estimated. As much as men bemoan how feelings are of little importance, they are actually the lynch pin for our manliness. I'm not talking about chauvinism or pigheadedness. Just clearly defined (workable) definition of what is important for him to be seen as a man.
I hope that makes sense. If it doesn't make any sense please check out our support groups regarding this topic. Other folks in the struggle might offer a more compatible perspective.