Did you ever feel like the big, bad parent with your cheater?
I think sometimes some engage in cheating behavior as a way to try and resolve family issues (non-therapist speaking here). They did not feel safe to do that with their mother or father. So, the faithful spouse gets all their rebellion in its full, awful glory.
When a faithful spouse tries to set a healthy boundary–like not dating another person–they take it as “controlling” and play the “I don’t have to” game.
Pain aside, this is a very sad thing. The cheater is clearly stuck and incapable of a healthy relationship.
I pity–but do not excuse–cheaters stuck in this cycle.
Their behavior is a manifestation of very deep and troubling spiritual sickness. If they are not willing to take the bitter medicine to make it better, sometimes it is better to leave them to it, though.
Sadly, I wonder if some cheaters were actually given the candy when they threw a tantrum in the store. Perhaps, tantrums worked for them for a long time.