@Delilah
Spoiler!
Jae Young walks into the house. He goes upstairs looking everywhere.
There, he hears Sang Woo talking with his mother. The conversation he hears is just too cute, and totally in their image.The mother totally accepts the relationship, to the point that it goes completely unnoticed between the subjects of conversation relating to his studies and his daily life. She's just jaded and factual, like her son, she asks him lots of questions. He tells her how he proposed to Jae Young by grabbing him with a ring, tells her Jae Young's handsome with pride, talks openly about his very, very busy s** life (he says he's aroused just by making eye contact ), reassuring her about about wearing condoms, etc.
His mother sees the changes in him: he seems happy and having fun. He confirms that his life has never been better than since he's with Jae Young and that he wants to keep that all his life. He says that Jae Young is sometimes lazy, but that he'll do with it, and he sometimes does things on purpose that will make Jae Young happy, he also had to make compromises (including seeing a movie once a month), he shows his mother videos of Jae Young skating, etc.
Listening to that, Jae Young laughs and is happy in the hallway. He also enjoys discovering a new Sang Woo through this mother-son talk.