Okay, so this is kind of the second half of a different story, and it might confuse lots of people if they don't know the beginning, so here's a brief summary of what has happened previous to the next chapter. But you don't really need to read it, everything is explained eventually.
All the Marauders have been hanging out at Sirius house when his parents are away, and eventually Peter and Remus fall asleep. At that point, somehow James and Sirius become intimate while hanging on Sirius' bed. They kiss and Sirius mentions that he has wanted to kiss James since they met, and James gives Sirius a hand-job. Then…