Remember that the archetypal NTR story isn't just about the girl cheating with another guy, it's about the guy corrupting her and breaking the original relationship. It's very much just what a corruption storyline looks like from the point of view of any of the other guys in the life of the girl being corrupted. Going from a loving mother to a pleasure-addicted slut who doesn't care about her family is what is meant by stealing her in this context. And yeah, while it can be fun to play as the corrupter sometimes, I've never got the appeal of seeing things from the point of view of the other guy.