It wasn't an "accidental affair", he was raped by a professor who he thought he could trust and who he respected.
I'm thinking more from Youngmin's point of view, he probably thinks he was so intoxicated that he lost all rational thought.
He was lied to by his predatory professor, and feels responsible for what happened. Guilt isn't an uncommon reaction, but it still doesn't remove that he is a victim. Jungwoo does go onto rough up the professor, but I'm not sure if the professor was ever reported to the police. It's a shame that a rare "normal" couple in yaoi, are facing so much prejudice from shitty nosy friends, family like Jungwoo's dad and even university professors.
Yes I totally agree with this, it's just that youngmin wouldn't actually realize he is a victim since he was lied to and thinks that he accidentally had an affair which would be such a horrible feeling
imagine trying to tell your lover that u had an accidental affair, even if you explain the real truth, u can't be sure that they will believe. It would be so scary but I still think youngmin lshould tell jungwoo before it's too late