Chunwoo doesn't owe anything to anybody. He doesn't owe reciprocation to Ho-In and he does owe monogamy to Taesoo. Ho-In is a whole ass ADULT. If he decided to pursue someone who already turned him down and proceeded to come on to him saying that he's willing to be a place holder then that's on him and no one else. Ho-In is not a child that needs feelings to be validated just because he has them. That's problematic.
He doesn't owe monogamy to Taesoo if he doesn't want it.
If he wanna cheat on him because is not happy with his relationship (and I don't agree with cheating btw) he can possibly do it.
If he wanna give Ho-In a chance based on whatever reason he can do it.
And yes, Ho-In is an adult who, I hope; he can understand when it's to much.
If he wants to try getting closer to Chanwoo he can do it unless he becomes to persistent and invades Chanwoo's spaces.
Um. He's not cheating on Taesoo as they are not in an agreed upon situationship with boundaries and mutual understanding. And apologies, I mean that HE DOES NOT owe monogamy to Taesoo, damn autocorrect. And lastly Ho-In has been doing that already.....being persistent and invading Chunwoo's spaces and boundaries...

I don't know why there's so much hating for the MC since Chanwoo he's letting him do his things
If you have feelings for someone you don't let another person kiss you /touch you/butt in.
I feel like he's going along with Taesoo just because he feels obligated towards him like he owes him their relationships for some reason even if he still has kind of feelings for him.
The fact that he's in a relationship doesn't mean that everything is okay and it's going well wich is why, I think, he let's the MC getting closer to him.