
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.zone/read-manga/the_monster_and_the_beast/
this story has essentially no direction, its like the author is making stuff up as they go, like how is she sick out of nowhere, and ML seem to get slap in every chapter, plus so much violence that's completely unnecessary to the plot.