Chanwoo wasn't in the right metal state after being used and abused, how do you expect him to even know of his feelings? He was nothing but a victim, MD was the one that hurt him to make him realise it.
So MD was supposed to wait forever for Chanwoo, while Chanwoo went to other relationships.
Even if MD didn't kiss that bastard in front of Chanwoo, if MD moved on with someone else, Chanwoo would feel jealous and hurt because then he would of realized too late. So no I don't see MD in the wrong.
But MD shouldn't hurt a distraught person, he was just used and abused.
I'm glad some people are actually on MD's side. I mean it is a little cliche that Chanwoo realizes his love by feeling jealous, but nevertheless it is a good read. Not gonna complain