Eeeh, that was great! Sirius/Lucius is one of my favourite pairings and I loved the way you told it all through James eversoslightly shocked and notjustalittle excited perspective.
Loved the Marauder-frame, and the way Sirius and Lucius use each other for their own ends is awesome. As is Sirius' challenging come-on stance at the beginning of the scene.
Great writing, too - my inner Lucius-lover squeed like mad over Lucius slicked himself as if preparing for a blood ritual, slowly and carefully and showily, even though no one was watching. But then, Lucius probably always thought someone was watching him, and more often than not would've been right.