Imo, just cause you can't be w the person you caught feelings for, doesn't mean you're now doomed to a life of celibacy. They're not together, there's no responsibility there for bro to be sexually monogamous. Plus it's not like he was leading his hookup on either. I don't see an issue w any of that.
The real problem is SAing his love interest at all, but esp after being traumatized by a bunch of mfs. And then SAing him again in S2 while pretending his memory was gone. That was so f'ed up. I would NEVER do that to anyone, least of all to someone I gave a shit about. And I fckn hate how the author made it all seem like Ai's love was so high value, that he was even willing to SA the MC in order to protect him. Like GTFO w that shit
Yes with your 2nd paragraph, but well we had different take with the first. Cause I will never be physically intimate with someone(even the other person was wiling) if I'm not in love with. Be celibate if I never find someone I love, then so be it. Would it feel good if you say, hey I f*cked around but you're the one I truly like. ┑( ̄Д  ̄)┍, I will not believe them, they're just in it for the body. Self control is one of the most important attitude you can have and give in a relationship.
Anyways, just my opinion.

I really don't get how one can just sleep around just because they can't get someone that they really wanted. This is just to satiate ones lust. Like, I lust for you but I cannot get you so I will be getting it on to anyone willing. That's why I called it lust not love. But moving on to love another is a different matter for me. And as far as I can feel, this is not the reason the ml is doing it. He's doing it not to forget but to release his sexual frustration. Because of this, he also forced the mc. I might have like the story, but this is not my cup of tea. If I am the mc, I would say, ah so you're okay with other people then get on with them, it doesn't have to be me ╮( ̄▽ ̄)╭
Ah I can't explain it well. (I know anyone would get me for this cause many really like this manga)