This change in the Doctor's behaviour wasn't exactly unwelcome, but given how apprehensive he'd been toward Jack's advances thus far, it was unexpected, certainly. When he broke the kiss, back bit his lip, trying to focus on what he was doing, but gasped when the Doctor's hand came between them.

"Y-yeah... Oh god yes I want that.." He swallowed, pushing into his hand instead. He wasn't sure exactly what the Doctor was planning on doing, but for the time being, he really didn't want him to stop. To continue encouraging this behaviour, Jack leaned forward to start kissing the Doctor's neck again. He did seem to have enjoyed that quite a lot.