Cheating is basically (falling off the wagon) whether it's a promise to be monogamus or stay on a particular diet. I suspect what these men who cheat mean is it is (human nature) to want variety in all things. However as with anything we as human beings always have a choice in what we do no matter what the urge is whether it's getting a second slice of cake or having sex with someone new. Anyone who has watched the TV shows "Cheaters" or "The Maury Povich Show" aka "You are NOT the father!" is aware cheating is not monopolized by men.
Monogamy is a life-style (choice) not a genetic code. Most people who get married don't plan on getting divorced or cheating. Quite a few people make promises without doing some soul-searching. Biting off more than you can chew does not make you a "bad person". It makes you an "irresponsible person". If you aren't sure you can commit to something it's probably best not to make the promise in the first place.
Regardless of what a man or woman for that matter says when asked (why) they cheated the vast majority of people will tune them out. In fact the question of (why?) really is a retorical question or "reflex question". They could care less what the reason, excuse, or justification is. It's almost pointless to get into that discussion if one has already made up their mind regarding the outcome.