I'm not sure if this is what you're looking for because I'm sure you would have found this one from tags or memories. It's O.G. Walton's Fantasy Series.

http://www.mags-nificent.com/MSW/OG.htm

Justin cheats on Brian with Tony who is even older than Brian and Brian cheats Justin with the same guy and then all the three of them end up having a relationship.

-LP-