I know, right??? When I read the novel, I was super furious. They basically sold seonwoo off to yiwon like their son was just some item, and don't care about his feelings, life or explanations - though they KNOW it's wrong, know how DANGEROUS and CRAZY yiwon can be with his mental conditions. Cowardcy at its lowest (especially from the dad, I remember him screaming and pressing seonwoo so much)

In chapter 33
Seonwoo's parents will eventually force him to become yiwon's mistress, as they feel indebted to yiwon's parents. Yiwon had told them that Seonwoo have feelings for him, leading them to assume that Seonwoo was to blame for making Yiwon unable to live without him; consequently, they insisted that Seonwoo must stay by Iwon's side and live with him even though he's married