@Tatherwood
Yes, you are completely justified slapping your girlfriend for cheating on you, it doesn't matter which gender is doing what. Whether they hit you back is their choice. But assuming that they slap you for cheating on them. I mean, what do you expect? Cheating on someone is a betrayal of trust, and it renders everything you've done with that person void. These arguments when someone is caught usually happen in the heat of the moment, and really, can you blame the victim for being furious and doing something in the heat of the moment? Besides, it's much more painful to be cheated on than to be slapped.
"Women can slap men if they feel insulted"
They weren't just being insulted, they were cheated on and belittled. It goes way beyond just playground insults. I don't think you really understand how bad cheating can affect the victim.
Sure, a slap is considered battery and you can get charged for that. I'm not saying that what the character did in this chapter is a good thing. I'm just trying to paint another perspective because cheating is no joke and especially when arguments because of this happen in the heat of the moment, it's completely understandable. Is it good? No. Is it bad? That depends on a lot of variables. Besides, just because it's legally wrong doesn't mean it isn't justified. Because like I already said, a slap is nothing compared to having your trust in someone betrayed and reduced to nothing. That's more painful and tormenting by a large margin, and there are a lot of people who've gone deep into a depression from that (from reading others' stories to having a close friend experience it).
of course, we're talking about an event in a very comedic and exaggerated manga so it doesn't really matter. but still, if you really want to analyze this situation, yes the dude deserved to be slap for not only cheating and belittle his ex's but for also playing it off as if it isn't a big deal.