When the initial spark in the relationship is gone, most men seek for it and when they can’t seem to find it, especially when their partner might not seem to cooperate, they look for it elsewhere I.e outside the relationship, with someone entirely new. This keeps them satisfied because it is fun and more interesting than what they have with their wives/girlfriends
Also, asserting control is a reason why some men cheat. If they seem to have lost control of their relationships, some men cheat just to show the woman who is still boss and keep her in check, silly but true. It reminds the woman who the boss is and depending on the situation, makes her sit up and revolt.
As much as they hate to admit, some men are just plain weak and cannot resist any form of seduction, so when other women come on to them, they have no form of resistance whatsoever and get into bed with them.
Some women also give their spouses reasons to cheat, when men realise they aren’t getting what they signed up for from their women, they tend to go elsewhere to look for it and there are plenty women always willing to give them what they so crave for.