I didn't find him annoying for reciprocating Yumin's love as a form of repaying a debt, even if he didn't genuinely love her. However, openly declaring this to the world, especially to his friend, is essentially insulting and taunting her feelings, which is completely wrong. Imagine that same friend visiting this couple, fully aware that he's just pretending to love her. It's surprising that Yumin still hasn't realized this. I can only imagine Yumin's life being full of disappointments, as he might feel that marrying her settles his debt, and he won't put in any effort beyond that. His lack of understanding of human emotions makes him seem very insensitive.
I knew I was fucked up !!! Cuz I don't find the ex annoying or anything!!! Like maybe yeah he is indifferent but um ok !!! Maybe it's cuz I know that most of the marriage r always kinda like that !!! And if the husband/ wife doesn't cheat first then it remains same . On the other hand I know it was cruel for him to do that to her, like yeah he's fucked up n he literally wasted fl's time (8yrs )