"Funny?" Sirius looked genuinely confused for a moment. "S'not supposed to be funny." Come on. He was funnier than that when he was being funny. "Honest." He took a step forward, reaching out for Peter again. He looked his friend straight in the eye as he spoke, and the look made it clear enough that the words weren't purely induced by the fog of alcohol. "I want you, Pete."
It spoke to Sirius' stupendously large ego that he believed telling someone that he, Sirius Black, wanted them would be a tremendous turn-on. You complimented people in order to have sex with them, and it was, quite frankly, the biggest compliment he could ever think of. It was also very true. Maybe because of the forbidden nature of it all, but Sirius wanted Peter more than any other person on earth right now.