A much-linked article in the Atlantic: Why Happy People Cheat - A good marriage is no guarantee against infidelity.
It's a strange approach to the subject, written by a therapist of some sort. Why does anybody break any sorts of covenants, agreements, vows, commitments, laws, etc., even when they know how destructive to conscience and good cheer it can be?
Of course it's because we are implanted with strong, relentless desires of all sorts. Food, lust, love, violence, competition, revenge, excitement. Raw life forces. You can call them emotions or instincts or whatever you want to call them. Most human desires lack any moral, much less bourgeois-moral, components and often lack any practical components. We need only refer to our fantasies to see that.
A more interesting topic is to consider is why and when people do not break any sorts of covenants, agreements, vows, commitments, laws, social conventions, etc. despite pressing desires. It's more interesting because it's the special human part. The monkey parts are no mystery at all.