I loved this! Plotty, hot sex scenes, and I really loved that you characterized Peter as something other than a simpering hanger on. That doesn't happen often!
I especially loved this little bit: "Listen," he said. Sirius looked at him with red, swollen eyes, and a red nose and wet cheeks. Strands of hair stuck on his face, and Snape, with a very tender motion, swept them behind his ear. *swoon*
Oh and...
"What are you, an asshole connoisseur?"
Well, he was best friends with James Potter so that's a safe bet Severus!