You don't understand BDSM relationship. Sigh. It's not romantic relationship ship in nature.
And MD wanted to continue their relationship after rejection. If he was mature and responsible he would end their relationship in civilised way. Not the unhealthy, toxic one
I'm not too sure on how BDSM relationships work but I know a little bit about human nature. I know if I was in Chanwoo's position I would not like to put myself in that kind of situation because of how things can escalate especially if the other person has feelings. I'm not saying MD is entirely right either because on that last panel he did hit him. But when someone has feelings it's not exactly easy to end a relationship on their side so I feel if Chanwoo had no intention of furthering his relationship knowing that MD developed feelings than he could of walked out on his own as well.
But you see Chanwoo is a sub. He trusted MD and his judgement. If MD wanted to continue then Chanwoo thought it's ok.
I get the strictly sexy times relationship that the guy wants, but it's not like MD doesn't have feelings. If you keep looking for partners and ignore someone who likes you while still having sexy times together with that person then they have every right to get mad. MD should honestly choose his last sub, but I know he's not. So if he's going to choose the guy then he should definitely make him fall hard so he can understand that he's just as much as an option than MD is for him.