I'm confused, so MC just cheats and thinks it's ok because his boyfriend will forgive him?...

Uh I think he didn't cheat. He's drunk that time and that guy take advantage on him. Also it stated there that he was just teasing him and nothing happened between them

That's still a bit much, why would he drink with someone he doesn't know that well, especially when he knows he blacks out from drinking, you'd think he'd be careful as a grown man.

This, I agree with you a lot.