His humor in the situation had slipped with his worry that Rose had taken the joking too far, but seeing that the Doctor is more ranty than hurt by it helps him get back in the spirit of the prank.
"You know me-- love to do, love to watch..." He folds his arms behind his head and leans back some in his chair, though mindful not to get to the kind of angle that makes Ianto roll his eyes. "What's this about not pretending you don't feel anything for us and not getting close to anyone before, hmmm?"