The brother in law made him believe his husband knew and didnt care. In the novel dongsik is a sex crazed guy who has never had sex because he's a beta that's into manly men in suits. The first chapter is literally him jacking off in his car to gay porn. Then he tries to save heesol who jumped off a bridge. The whole novel and Manga is just fictional smut. They always do crazy shit in smut novels.

Why is everyone so puritanical lol. Theyre acting like having sex on stairs is the worst thing people can do. Theyre literally in their home tf