
I don't know why many people are confused or upset, but ms Kim has sex with her ex in order to put a definite end to their story. While having sex with him she thinks about the other guy and she realized she was free from her leash (which is her husband) exactly because she understood she cares for the other guy. I think we will finally get some good development for the main couple.
This is so sad. Sometimes we sacrifice so much for the people we love thinking this is the price to pay in order to be loved. But real love doesn't need a price, real love is all about freedom, letting the person you love achieve their dreams and do what they like. If they force you to do something else or don't notice how painful it is for you, then it's not love and you better run away from them.