I have not started it yet but from what you just describe it is indeed sexual assault/rape because the other party is forced and thus not consensual
Just want to clarify something, blackmail is "the action of demanding payment or another benefit from someone in return for not revealing compromising or damaging information about them" and extortion is "the practice of obtaining something, especially money (but in this case sex ) through force or threats" so technically Yiyeon didn't blackmail him he extorted him, but I don't mean to offend you by saying this, I'm just sharing.
Yes I do agree Yiyeon is a rapist, the only reason Lee Nan even agreed to have sex with him was because it was more like a necessity so he could have a job, he didn't do it because he wanted to, more like because he needed to (meaning he was forced).
OH OK THANK YOU YES I THOUGHT WHAT I SAID DIDNT SOUND RIGHT
He practically blackmailed Lee Nan syaing that he wouldnt let Nan find another job unless he hasd sex with him. Is this not blackmail rape? Lee Nan wouldnt have been able to go to the cops since he had no evidence and even if he did the guy hes up against is practically a chaebol. Does this mean that my homeboy Yiyeon is a rapist? I genuinely gotta ask because im not eductaed on the standards of blackmail rape