Wow, that was powerful! The Marauders as right bastards is one of my guilty pleasures, and you certainly didn't pull any punches during the initial non-con scene. It doesn't even feel very personal, more that James, Sirius and Peter found someone they both loathed and thought they'd get away with abusing him for their own... curiosity, more than anything. Very chilling and effective.
Loved the Remus/Snape encounter and bonding (I tend to be creeped out a little by werewolf!sex, but it worked extremely well here - the comforting aspect most of all, because it stresses the contrast between Remus (instinct and protectiveness) and the other Marauders malevolence.
Nice resolution/bonding too - excellent story throughout.