Responses
I know of which manga are you thinking but I also cannot remember the name but I know it's still really popular so you maybe find it in the yaoi list if you remember the look of the front page and a part of the name.
It's Kocchi muite ai
+RoseMary got it right
http://www.mangago.me/read-manga/kocchi_muite_ai/ (
づ￣ ³￣)づ here you go
I'm looking for a manga it goes like this one of the guys is in love with his close friend and lets him use his apartment to have sex with his girlfriend, he can never seem to refuse. And his neighbour is a right playboy. One night when his best friend is using the apartment he feels depressed and has sex with his neighbour and the fall in love