I am so confused

For the MC to feel properly satisfied he needs even more extreme play than what he's doing heewon and is annoyed because he doesn't want his wound to heal quickly or anything like that. And he just has a strong libido I guess.
In heewon's case, at the beginning they had decided that when he calls wookyung as Mr. Cha, it would mean that they are in a play. A reason why he switches from calling the mc by his first name or the formal way is, because he may be developing feeling, yearn for a closer relationship, where they get to know each other more and become something more than just a s/m hook up. He might not even be doing it intentionally, maybe he gets too engrossed in the act and forgets to be "professional".

Thanks for your insight. It isn't portrayed clearly between their messy interactions. I hope they finally come to an agreement bc the drama is getting tiresome. Was fine the first half but dragging it on has brought more confusion than anything else. Like I feel MC doesn't understand their relationship while ML isn't accepting what he actually wants. Leaves a bad taste where they should separate.