
I'm glad he got the crap beaten out of hin

Who got the crap beaten out of him? If you're talking about the black haired guy Natsuki was the one who instigated the sex and consented to it all the way through. If you're talking about Natsuki, this could be categorized as a spoiler, for one, especially since you DON'T mention who was beat up, and it proves you aren't reading the story. What about Arata? He also did something questionable with the black haired guy at the bathhouse at the very beginning of the story or does no one remember that?

I mean, leaving the name out sounds like you were trying not to spoil it even though at the same time you mentioned an event that doesn't happen apparently in the translated chapters. So just thought you might want to be aware of how not to spoil in future.
Also read below. Ktbn.

All Arata did was talk to the guy in the bathhouse, get his name and viewed him as a friend not really comparable to actually cheating on your partner multiple times. Arata didn't behave "questionably" at all, he still stuck to his boundaries unlike Natsuki. Him sleeping with someone else didn'y justify getting beat up though, and knowing that they have been together since they were kids and are now at this new stage of their lives then it wouldn't be unusual for them to grow apart and split up. None of this justifies Natsuki's behaviour whilst still in a relationship though, but going on the raws the two MC's stick together despite the cheating and the violence.
I trully hate the second story with a passion