I've been looking for a community like this for a long time, I'm glad I found you and I hope I can help, too.
I've been re-reading my favorite qaf stories, and I can't find one I really liked. The story was about Justin deciding to be monogamous because, as he tells Brian
"I never get off with anyone as long or as hard as I do with you" (not an actual quote obviously, just taken from my memory). Brian is mad at first, then he also decides to be monogamous and he breaks the news to Justin by repeating the same words (above) to him, and making love to him without a condom because he had abstained and had been tested.
I think the last line is something about Justin looking 17 again. So, I guess it's an established rrelationship story, set in the future.
I remember it was really good, sorry for the long description, I hope someone will be able to help!
Thanks.
Found in record time! It's Paradigm Shift by bigboobedcanuck