I think I follow what
@syze6 was trying to say here.
Every chemical reaction that attracts you to another person is still active while you're in a relationship, it's a matter of you choosing to ignore it, or not.
I'll take it a step further and say that weather you ignore it or not is based on "risk vs. reward" rather than dissatisfaction with a spouse.
Now dissatisfaction can be a huge factor in deciding that cheating is worth the risk, but most of the time, it's the reward of
physical fulfillment outweighing the risk of being caught that prompts the action.
There are fewer risks socially for men and they are not socialized to believe that fidelity is the truest expression of love. We as women in America have raised cheating up on a platform of "WORST VIOLATION EVER". Men don't see it like that (for themselves lol). And once you do something the first time, it's much easier to continue doing it.
So you don't need a "reason" beyond feeling an attraction and having an opportunity to fulfill it.