is it just me but i've been skeptical about the seme since he was first introduced, idk i ...

It's probably due to the way both of them are reacting to the difference between them in sexual experience and awareness. Both have a reason to suspect the other might not end up taking them seriously: Geonwoo's afraid to be seen as too childish for a relationship, and Yuan's afraid Geonwoo might be mistakening the novelty of sex for real love.