'the cheater’s paradox’

Let’s say a girl is attracted to some boy who has a girlfriend. and then the guy with the girlfriend decides, like, What the hell? We’re not married, We’re just hanging out. I can hook up with this other girl if I want to. It seems obvious to me that any self-respecting girl would realize that the guy’s decision to cheat on his girlfriend would make him an undesirable person to hook up with, right? And the guy who wants to cheat should be turned off by any girl who is willing to hook up with someone else’s boyfriend. Being so, morally bankrupt should cancel out all the attractive qualities that tempt you to cheat..right?