I don't think it's because he don't know what rape is, but the fact nobody think they can be a victim of rape. A big part of it also come from victim shaming. He knows it was wrong of is teacher to do this and he feel awful about it, but "he shouldn't have been drunk in a house alone with him and seduced him" <--(not my thought, to be sure i'm not misunderstand)
I also don’t think he remembers the part where the prof forced a ton of alcohol into him. If you leave that part out, Youngmin got drunk, thought he was with Jungwoo, and asked prof to have sex. I find this much more realistic than most other rape manga. The prof knew he could get out of it by saying Youngmin asked for it and leaving out he forced alcohol down his throat.
Does no one in manga know what rape is? I'm not criticizing him, I'm just genuinely concerned that he doesn't seem to understand that someone having sex with you while you're passed out drunk and think they're a different person is a crime.