i suppose it could be because he's a bystander. he also might've been harassing yuwon in his own way. at least that's what i figured with how seongmin is seen casually gossiping w coworkers; and that weird exchange sequence about yuwon being a deep sleeper. he implies that it'd be easy to take advantage of drunk yuwon in such a vulnerable state. it's played off as something innocent, but it's not like he would admit to something like sexual harassment outright, so i see where the suspicion comes from. also it had been shown how yuwon can misjudge people or their intentions (e.g how he refers to his past bullies as friends; which also can be a trauma response, but two things can be true at once) so he can't see past seongmin's exterior

as soon as i was thinking seongmin was innocent……. nobody is safe