Im pretty sure its this one
http://www.mangago.me/read-manga/the_prince_of_proctology/
Whyd this make me laugh-
It's not hahahha
the first manga i thought of when i saw your question. hope it's the one youre looking for~ http://www.mangago.me/read-manga/shiri_fechi_nanka_ni_sukarete_tamaru_ka/
If not the ones above, then maybe check out the tag?
http://www.mangago.me/tag/Butt%20fetish/
Yesyesyes thankss ♡♡♡
I've been looking for this yaoi where the uke has this big juicy ass hahahaha and you can say the seme fell for the uke because of his big booty (￣∇￣")