Jack flicks a quick grin up at him at the reprimand, but it brings more heat to his cheeks too. He loves being able to push, knowing that Ianto won't let him get away with anything, even if he doubts any actual punishment will follow from this. Ianto's clearly enjoying the show too much for that.
He lets out a breath and pushes the plug in another few millimetres. Not far to go now, thank goodness, but what's left is the widest part. He concentrates, focussing everything he's got on the muscles of his ass, as he slowly jiggles the plug in and out the tiniest bit, working that last necessary bit of stretch out of them.
And then the plug finally slides in, the flared base of it nestling against his ass, and he gasps and pitches forward against Ianto, panting hard. "Fuck!"