I think its YW because throughout the series YW makes it a point how Seo-In treats sex in general (including advances, oral, etc) as literally nothing more than just…sex. To YW he views "sex" as something that matters to him, because he wants to do it with someone he likes.
One night stands aren't his thing. YW is asking himself, "how many times do i need to continue doing this until these thoughts of [sex is suppose to be meaningful with the person I like] turns into [sex is just another activity, it's just sex and nothing more]".
Seo-In, who he meets now, is different from the Seo-In he knew when they were friends 10 years ago. His sudden switch up confuses YW because the Seo-In now is suppose to be someone who doesn't care and does whatever he wants.
YW is prob confused as to why Seo-In is apologizing and acting like he cares. Seo-In apologizes because he's afraid YW will suddenly leave him again, and he can't go through that again. The sudden change confuses YW, and prob hits the soft spot YW has for Seo-In.
I know they like each other still, and Seo-In is prob acting like a jerk because he's angry and hurt YW abandoned him without saying anything. Plus, in ch 28, YW says "it's better if you keep talking like that, that way I won't have any expectations".
Got confused who's pov was the ending part