I'm not sure I understand the Remus is this story. Is he treating the seduction of Severus as a game in which he tells all the details to Sirius? I'm wondering because of these two lines:
he whispered dutifully and kissed Snape's head.
Before he fell asleep again he thought of how he would not hear the end of it when he would tell Sirius about that night.
This doesn't sound like a very kind or decent Remus.
Anyway, I'm interested in seeing where this story goes. I'm glad I only have to wait until the end of the month ^_^