Because while you’re cheating you’re concretely interacting with real people. You are entirely exposing yourself to real people, you are not watching sex characters on a screen. I would surely be upset if my partner watched porn, but it’s not equivable to real cheating at all. Plus, I’m pretty sure that, if this situation had reversed genders, nobody would have said “Talk to him, don’t dump him, even if he has cheated on you”. There are also a lot of biological factors that play the role. Not excusing cheating, but if a man cheats it is, in most cases, because of physical sexual needs, while for women it’s way more of an emotional response.