It's not the movements that make the sex. No, it's not the fast and frequent thrusts, or the languid strokes over the head of his cock, or the flick of Jack's tongue against his shoulder blade.
It's none of the above.It's the fingers running through his hair as they kiss. It's the way Jack's head lolls backward and his mouth opens ever so slightly when Ianto does that .
It's also because afterward Jack pulls him tight to his chest, and Ianto can feel his Captain's lips mouthing those three little words against his shoulder.
Always mouthed. Never said.