Sirius blinked, since he hadn't even been close to meaning it like that, he'd just been having a go at her. Sirius usually said the same sort of thing to people buying HIM drinks, that he needed at least three before they could try to talk him 'round to sneaking off to the loo or a room for a quickie.
She was gone before he could argue, and he just shook his head, pulling out his own wand and casting a drying spell. "This," he informed Tom, "is why I usually shag blokes." Tom looked unimpressed and Sirius added helpfully. "But not you." He paid for the drinks and slipped off, wandering back toward the shop, simply because he didn't feel like heading home.