Changsub isn't bothered by the fact that Ilhoon is gay and living with him (in fact, he's gay himself). What bothers him is that no one else knows but him. The affection Ilhoon receives from Sungjae makes Changsub's mouth twitch and his hands itch. Ever since debut, Changsub had been keeping his eye on the youngest. Changsub never bothered to approach Sungjae because he knew he'd fall even harder, and if this was the game Ilhoon wanted to play, Changsub wouldn't hesitate.