i remember that the Seme is a demon i think, hes huge and he can hide in the Uke shadow, he think he's ugly but he's adorable and the Uke is a middle age man, but he's very seductive and like to sleep around, this is a manga.
This is what you're looking for i think http://www.mangago.me/read-manga/the_monster_and_the_beast/
did the art change? haven't seen this in a while, i prefer the first few chap art