Well, the real abusive person is actually someone else. Sure, Seungho is no saint but the one who brainwashed the young man is the teacher. What he did is called coercive persuasion and used brutal techniques to destroy and manipulate the painter. That's why the young painter is physically so weak. The real sick one is Jung In-Hun because in my opinion, he is suffering from a narcissistic personality disorder. So if there is any Stockholm Syndrome, then it was with the low noble and not Seungho. The sex session in front of the mirror actually had a positive effect.
it was rape. it doesn't matter if it has a positive effect or not. it was rape. they are both abusive. and it wasn't brainwashing, it was teaching, without a genuine intent for brainwashing. but then he took advantage of the admiration. both are abusive. both are the real abusive person. it's not a competition. the only time that there was ever any consensual sex between the two MC's is when the ML is heartbroken and not in a proper mental state.
Oh it absolutely is abusive. But it's a period piece. We have the privilege of being aware that this stuff is wrong now, but at the time of place, there are very different leverages leading to this indeed being somewhat "romantic." And they won't ever become a couple. At the very most he'll be a mistress, probably one out of many, but a favored one.