Mokka, Mimodaeshi Mune no Uchi.

• Meshita Mimodaeshi Mune no Uchi Gen was lying bruised and bloodied on the sidewalk when Kiba found him. The two became friends after that. One day Gen was surprised to find out the girl he was sleeping with was Kiba's ex, but hearing about his friend became an obsession as Gen picked up more and more of Kiba's "leftovers"...

• Mokka, Mimodaeshi Mune no Uchi

Source: MangaUpdates