Jack even defers to Ianto's opinions on things. In "Meat" he asks him if he really shows off (to which Ianto says he does, and Jack sort of shrugs and accepts it). In "To the Last Man", Ianto initiates the mega-kiss. In "Adrift" Ianto is the one getting Jack off. AND, Ianto is the one to go behind Jack's back and do his own thing in regards to telling Gwen, and it's allowed. Jack knows it's Ianto (he says as much to Gwen) but he never punishes him. From everything series 2 as shown us, at least from what I have seen, if there is a dominant one in the relationship... it's Ianto and not Jack!