The Master reacts as expected. He bucks his hips against the Doctor, almost as if trying to throw him off--which he would have managed if the Doctor hadn't known this was coming. As it is, the Doctor gets up on his knees, resting his weight on the mattress rather than the Master's body, and balances himself with one hand while his other stays on the Master's buttock, just resting loosely--just touch, a reminder that he's still here, and that they're playing.

It's just a game, Master. Calm down.