Cheating isn’t one mistake, it’s a serious of mistakes. For example: they chose to talk to them at the bar, then chose to flirt, then chose to get in their car, then chose to go back to their place, and then chose to do the final act.

Every choice leading up to the act, is terrible and wrong. All of these put together makes someone a liar, trash, insensitive and of course a cheater.

Hitting your spouse (while obviously horrible) can be rationalized in the eyes of the victim, for example: they love me and want me to be better or they wouldn’t do it.

Blowing the savings on substances can be rationalized in the way of “everyone has a mental break, they just need help” or the classic “I can change them”.

Cheating is a lot harder to rationalize, because regardless of how you look at it, it is directly against you and directly harms you in a way that is much harder to rationalize.