I believe the story is told in alternating POVs. Justin goes to L.A. and during his stay has a drunken bareback encounter (maybe he finds out the guy is positive? I can't remember). He doesn't want to tell Brian about it, but when he comes back to Pittsburgh and won't have sex with Brian, Brian thinks he's cheating again and gets upset. I think that in the end, Justin tells him what happened. He gets tested and doesn't have HIV. I read it over at Archive of Our Own. I can't remember if it's a recent story or not. It was super well-written.
ETA: Found! The story is
From a Safe Distance by bigboobedcanuck
P.S. Mods, I forget: Do I do the tags or do you guys?