To Break-Up or to Cheat? That is the Question.
I'm looking for a fic wherein Remus, because of his affliction, is driven to have sex just before the full moon, and chooses breaking up with Sirius (who isn't having sex with him) over cheating on him, as the lesser of two evils. The fic starts just shortly after the first of these breakups with Sirius in a sorry state and I believe James telling Remus to "fix it".