Read it looong ago. The plot was something about a guy who was in love with his friend from college so he let his friend and his girlfriend sleep together in his flat. The protagonist's neighbor also goes to the same college and finds out the protagonist likes the other guy and screws him while the best friend is screwing his girlfriend in the next room.
If anyone knows this pleaaseee tell me
http://www.mangago.zone/read-manga/kocchi_muite_ai I'm pretty sure it's this one.
I literally get heartsick everytime I read this series ITS TOO CUTE