Ok, so let's have some summary from other comment here ->
" Hyun Woo, who's studying in Canada, returns to Korea for his older sister's exhibition. At the exit of the airport, he meets his ideal man...who turns out to be not only the manager of the gallery where Hyun Woo's sister is holding an exhibition, but also her betrothed...But he is about to find the darkest secret this dazzling man is hiding behind his pretentious smile..?? "
Mind you, Hyun woo is gay and he knows it. He ain't shy about it nor feel guilty about it. (although he's not like openly obviously gay, but you could see the gayness).
The fiance looks nice, kind, and proper. But just like the other comment said, what lies behind that smile? He insisted on having sex with the younger brother when he's betrothed to mc's sister??? ummmmm
Spear of Purgatorium Webtoon