I agree with a previous comment.

It looks to me like Sangwoo is competing with Bum's uncle and overwriting Bum's "firsts", hence "i win". The throw up is actually already explained before it happens, there was another time when he was propositioned with giving someone head. If you remember, there was a gay guy Sangwoo seduced up until the guy pushed down on his shoulders to get Sangwoo to give him a blowjob, which is where he lost his composure and attacked him and took him home to kill. In that scene, i don't think he intended to kill him, he had only seemed to go after women and he went to the gay bar after having thoughts that implied he was confused about his relationship with Bum. He went to see if he was into men in general as well as women or if Bum was just confusing his dick. Anyways, you see in that scene that he's really uncomfortable in that position and maybe at the thought of giving a blowjob in general. Sangwoo, the master of camouflage, actually lost his composure and knocked the guy out. It could be a trauma or just a preference, but he alludes to his discomfort while in that position in this latest chapter as well when he says "that's not it. I might be mean to you, i have trouble controlling what i say". He was worried that what happened after the scene would happen during the blowjob, that he'd get disgusted and say so to Bum/throw up. I think the fact that he almost tried for Bum tells way more than the puking.