It was Tonks' suggestion that Sirius' boredom was getting the better of him, that he needed a little adventure lest he find himself in trouble, that led to Bill's dare.
"I can make her come faster than you ever could. You're out of practice."
Sirius' eyes blazed with the challenge, and Tonks felt herself melt in response to the promises she saw there.
"Every drop of you," Sirius panted, thrusting his tongue inside of his cousin, "I'll have every needful drop."
If Bill hadn't have been holding her down, she'd have fallen off the table from her thrashing.
"So . . . good!"