South Korea had a case where someone claimed being drunk as a defense. They raped a child and used to fact that they were drunk to lessen their sentence, and well it worked because being drunk (at least during his sentencing) is/was by law a defense. So yeah its a little iffy on how rape and alcohol works in SK.
About the American topic, although its common between partners and friends its usually either dub-con, abuse, or they cant take a hint. As someone who was almost raped by my little brothers tutor as a kid and who's mother is a rape victim, it depends on the person, the situation, and what state/city your in. The averages can always change. If that helps at all @karrot
Genuinely concerning how many people are arguing this wasn't rape... I can't speak for other countries, but if you're American, by American standards this is definitely rape. It doesn't matter that they were both drunk, or that they liked each other, it's still rape. What makes this even worse is that because this is a flashback from Hwan's perspective, we know that he remembers this happening. However, because this was never brought up earlier in the story its pretty safe to say that Chiwoo doesn't remember, which is a testament to how inebriated he was. It literally doesn't matter that they liked each other or that Chiwoo appeared to enjoy it. In fact, correct me if I'm wrong, but I think in the US most instances of rape occur between close friends and partners than complete strangers. Maybe I'm wrong for applying American laws to a South Korean work, as I have no idea what the laws regarding consent are over there, but Chiwoo could not consent as he wasn't in his right mind. It does not matter that Hwan couldn't consent either because he's the one initiating, him being drunk does not make this not rape. Anyways, I'm not saying it bothered me, but I think it's 100% valid for people to have been disturbed by this especially since he seems to be endgame.