maybe to him the one night stands are nothing special because muyeong always moves to another person the next day. sooo he thinks he's hot shit, knowing that he's special because he's been with muyeong ever since their school days. but then seonho came along and muyeong's starts crushing on him... makes him feel a little threatened, i suppose. that whole kiss in front of seonho was probably him feeling superior, and sorta staking his claim as the 'fake bf'. it wasn't necessary to kiss the guy, a normal proclamation would've sufficed.
Frfr. He really stepped over Muyeong’s boundaries with that one
I feel strange. If his friend loves him, while he feels ok when he have a one night stand with someone.