Since I have only done it once, I am not an expert but I think cheating and leaving are two entirely different things. I think cheater don't want to break up. Leavers want to break it off.

There are different kinds of cheaters. The philanderers and those that can't help themselves.

I was not looking to cheat and certainly did not want to end my relationship. I was seduced by a past friend. She did not have to try hard but she was definitely the initiator. It was easy to go along and hard and awkward to stop it. I actually did kind of put a limit on the activity but it certainly was something that my girlfriend at the time consider infidelity.

Access, ease. That is why some men cheat, because it is there for the taking. Yes, if your sex life is good you are more likely to avoid being seduced. It is easy to say no when you have a hot girlfriend/wife and are having sex as often as you want. Even if maybe your girlfriend/wife is not really hot and sex is not as often as you would like you are still maybe not ripe for seduction.

Some guys don't cheat because they have no access to other partners or it is hard to find one. Some guys can't get laid if their life depended on it. If you marry someone that it is attractive to a lot of potential partners, well, you might have to be very good to them if you don't want them to cheat or you might need to let them know that if they cheat it is a really big bad deal.

Bill Clinton and Donald Trump were destined to be cheaters. Their wives knew it.

Motive and opportunity.