Kurt and Sebastian are best friends and roommates at Dalton, and are very close/affectionate. Kurt and Blaine get together, but Blaine gets jealous of how close Kurt and Sebastian are. There was a scene that I remember pretty well: Kurt and Sebastian are outside during their lunch period and Sebastian is massaging Kurt's back and kissing his shoulders/neck. Blaine sees this and gets super insecure/jealous and lashes out at Sebastian. Sebastian gets angry back and makes Kurt choose between them (asking who is in the right) and Kurt chooses Blaine. Sebastian storms off, Kurt comes to him later and apologizes, and they make up.
That's all I really remember, does anyone know what fic I'm talking about?