In response to grey_bard, I have often considered this very idea and wished it were canon. Whilst there are moments where Ianto is clearly in control (the stopwatch scene, Ianto mocking Jack's interrogation routine in Sleeper, and various quips in Something Borrowed) I don't think there are any suggestions that Jack might not necessarily be The Best Sex In The World Ever.

Surely Jack is too interested in himself to be outstanding in bed?