Sirius didn't trust his legs at this moment, so he did as his heart dictated - he laid his head upon Remus' knee, and wrapped his arms around the other's boy's leg and hugged it tightly. Remus was his anchor, his Rock of Gibraltar, his guiding light. He didn't know what he'd do if he should ever lose him through some stupidity, or for any other reason. His relationship with James, although tight, was just not the same. Though he could not have said just why that was, or in what way it was different.
And then the words came through to him, what Remus was saying, and he wanted to protest, to ask Remus why, but he choked it down and swallowed it, forced himself not to react first, for once. He closed his eyes tight, praying for strength, before opening them again and rising from his knees, disentangling himself from Remus.
He rose to his height, which was not yet his full height, but almost so, yet still under that of Remus, and said in a serious voice, "All right, Remy, I'll do it." And he meant it, every word. And Remus needn't worry about Sirius going onto his knees for Severus Snape, that was never going to happen.
"How'd you want me to do it, then? I can't very well go into Slytherin, obviously. It doesn't have to be in front of other people, does it?" Somehow he suspected the answer to that was no, but he wanted to hear it confirmed. It would be humiliating enough as it was, doing it at all, but that much worse with witnesses.