I've read a story where Justin falls for this guy. He cheats on Brian, the whole nine yards and Brian finds out, but he continues to do it. I remember Brian getting shot at a big mansion and whenever Justin fights with Brian, he'll run to find the other man. The man also describes Justin as a 'feisty cat' and a lot of things like that, and I believe there's a perfume named 'Justin' in it, while the guy is a client of Brian's.
I believe there was a line going "If you let him go the next time, I'm not letting him go back to you," or something close to that vein. It ends in Brian/Justin, with the man going overseas, I believe.
Thanks for the help!
ETA: Found!