Jack's shaking so hard now he knows he can't keep this going much longer, however amazing it feels. His climax has been building for a while now but he's trying to hold it back, because he wants to make this last. Partly because he's a masochist, and partly because it's a first with Ianto - and a fucking fantastic one at that.
His mouth finally slips from Ianto's because he needs to pant for air. He presses his forehead against Ianto's instead, eyes closed, just feeling.
((He totally did! Or, you know, everlasting perfect make-up is just a part of the conversion process. So they look fabulous underneath the metal or something.))