It was something Sirius had intended for only a moment. Moony had speared his own paw with a large splint and Sirius could see no way to get it out except as a human. Sirius planned to transform, pull it out, and transform back again before Moony could notice. He knew it was dangerous, but compared to watching Remus bleed and howl in pain for hours it seemed a small risk.
Later, he would try to convince himself that he was simply lucky he hadn't been bitten or killed. Later, when he couldn't sit or walk and refused to tell anyone- least of all Remus- why, he would tell himself how much worse it could have been.
The moment he transformed into a human, the wolf was on him. He was sure he was about to die when he felt himself flipped effortlessly. He was already naked- not quite adept enough at the Animagus transformation to manage transforming his clothes just yet. The wolf was not gentle. It's large cock entered him swiftly. Sirius could do nothing but scream.
The next half hour was sheer agony. Sirius did what he could, fingernails clinging to the bare wood of the floor as the wolf entered him roughly again and again. He felt the friction ease and would only discover later that it had done so because he was now bleeding. Nevertheless, he was grateful for that lubrication. The wolf's claws swiped at his back, sometimes holding him down, sometimes lifting him up. Sirius's own body betrayed him, his cock hardening under the assault. He came a second before the wolf.
Sirius felt the wolf come inside of him, and thought it was over. He had not anticipated the tie. It wasn't worse. Nothing could be worse than what had already happened, the pain of what had already happened.
When it was finally over, Sirius stayed. Just before dawn, he left the shack. His first prayer was that Remus would never realize what had happened. Remus would be devastated.
His second prayer was that it might someday happen again...