Roommate sleeps with girlfriend and relationship continues over the years

Looking for a specific story that I think might have been removed from literotica. The protagonist lets his girlfriend start sleeping with his girlfriend late one night, and this turns into a continual relationship. I remember one part where the girlfriend and friend have sex outside at a party and the protagonist is humiliated by people realizing that. The girlfriend and friend start going on dates even after the protagonist marries her and I think she eventually leaves him for the friend. Any help greatly appreciated!