In the new chapter! Spoilers alert! MD says he only did that to see Chanwoo's reaction. That's shitty of him, Chanwoo was in a state of distress and was extremely uncomfortable.
I know. BDSM is partly about "I tell you what to do and if you don't you will be punished". Also, he did way 'worse' things to him than that slap, think about his badly bruised butt. That there were feelings involved that made the situation very uncomfortable is nothing I want to disapprove, though slapping is and always will be a part in those sexual plays. Sorry man.
Of course he did that. I didn't wanna say that the situation in general is okay, because it isn't. It's just, the whole setting is fucked up, not the slap, the slap is typical BDSM-ish punishment.
First of all, Jiwon and DG are so pure and adorable and wholesome I could cry. He really did something out of himself there, it is just wonderful that he reflected on his faults and his such a nice bf now.
About MG and Chiwoon... the slap was obviously BDSM play. That's what you do and if that is too much for the sub, they can say the safe word. I don't see how it's that dramatic. On the other hand both of them acted kinda stupid, maybe even naive. Both of them don't have positive experiences which is why they probably don't have a way with words. After the last chapter I am sure they will manage things and it will even be as healthy as Jiwon x DG. Some relationships have a complicated start and that's what I love about this story. Because it's real. And sometimes things have to be shitty in order to become beautiful.