The second time they slept together, it was because of Remus' lycanthropy. [Fourth Year, the Shrieking Shack]
It was a cold evening, just as the full moon began to slip away. Sirius looked up into the star dotted sky. There were still there, but faintly. Sirius guessed it was around 5 o'clock, and looked back at his shuddering werewolf friend. This hadn't been the first night he'd snuck out to see his closest friend, but it would be the first time Remus actually saw him. It really isn't fair, Sirius thought to himself. No one should have to ever suffer through this, especially Remus. Especially since Sirius had gotten his favorite shirt caught on the door. He had to toss it off, and there it lay. Remus' wellbeing was worth more to him than that shirt, anyway.
The said boy whimpered, and Sirius bent down and gathered him in his arms. He carried him off the floor onto the mattress in the corner. Remus was sound asleep, or if not, he didn't say anything about Sirius carrying his naked body to the tattered mattress and lying down next to him to keep Remus warm. Suddenly, he found Remus to be much warmer, but couldn't move away from the heat.
Though Sirius didn't know it at the time, Remus was wide awake, and was blushing at being pulled flush against his best friend's bare chest. When Sirius scooted closer to his body, he tried to ignore how snugly they fit together. He tried to ignore Sirius' sigh of contentment, and how his own heart was beating twice as fast as Sirius', yet perfectly in time nevertheless. He tried hardest to ignore all the signs that pointed to him being in love with his best friend.
Eventually, Remus stopped trying to ignore what his heart was telling him.