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If Lee were going to do something so disturbing, he would have said sleeping with his father or specifying that it was a kiss on the mouth. But he said kiss without specifying what kind of kiss and where, because he himself is not able to visualize something inappropriate with his own father. The author assumes that we all already know the personality of the main characters, especially Lee Won's way of being, to be able to make a joke like this, but unfortunately it seems that people do not understand his sense of humor.