Still haven't been able to find this one shot, so here goes:
A while back I read a yaoi one shot where this guy runs into his childhood friend he hasn't seen in a while. The MC is with his boyfriend, and the childhood friend (CF) is with his girlfriend. The MC was always open about his sexuality. Apparently, they were good friends in high school, and the MC refers to CF as air. Anyway MC finds out boyfriend cheated on him with CF and that CF was always in love with him and then they sleep together.