I wouldn't even bother with these sort of post to be honest, I see the same incel level logic on social media all the time. People always making excuses for men cheating, and the same dumb reasons why women cheating is worse. Usually their goto is that men don't catch feelings when cheating... however that's not always the case, I've literally witnessed this happen IRL. Also, it's not always the case women do either, women have cheated without catching feelings, many times, it's not black and white.
Regardless of catching feelings or not when cheating, it's WRONG, end of discussion. No matter how much anyone tries to justify it. There is a very rare exception when it comes to someone who's married (and can't immediately divorce), and it becomes abusive, so they find comfort with someone else, that's like the only circumstance where it can be justified imo.
I do agree that in the context of THIS game, that yes, Mary is worse with her cheating, because it goes beyond just cheating from what I've read. She actively tries to humiliate him in real time, and hurt his as much as possible in that moment, which is something the MC didn't try to do to her. So with that in mind, I do think that Mary IS the worse of the two in the context of this game, with the circumstances in front of us. Apparently if you only cheat on her once, she forgives it and doesn't revenge cheat, but even if you find out someone cheated on you twice, instead of humiliating them and purposely hurting them, she should have tried to reconcile first, and maybe take a trip to their therapist instead of taking such drastic measures. (I'm not sure if they do go to the therapist or not, but since I'm pretty sure she cheats no matter what if you cheated at least two times, I assume it doesn't really matter.)
In conclusion, both are firmly in the wrong for cheating, it's not right under almost any circumstance, however yes Mary is worse because she goes beyond just cheating, and actively wants to hurt and humiliate the person she's supposed to love. While the MC, who's still in the wrong, wasn't trying to hurt Mary in the process of his wrongdoings.
I'm never going to play the cheating path, because fuck cheating, I much prefer the happier and more loving side of things. Drama still finds it's way into the plot, but it's usually external factors out of the couples control, and they get through it. I will continue to play the swingers/hotwife path, that's where I find the most joy with this game. Then I will likely replay and do the pure romance/monogamous path.