"Oh, I bruise," he assures him, smiling as he watches Ianto go loose and happy beneath his fingers. "You just have to hit me hard, and a lot."
He draws one fingertip down over a particularly dark bruise, watching the trail it briefly leaves behind disappear. "So yeah, a little envious that you bruise so easily, so beautifully. But more, I meant... doing this with you, it's making me realise how long it's been since I was on the other side of the equation. How much I miss it."
It's not in any way an attempt to guilt-trip Ianto into beating him, considering how much emphasis he's put in the past on not wanting to see him hurt. It's much more about opening up, as Ianto is, just sharing how he's feeling for the sake of sharing. Rather enjoying the novelty of being able to share this with him, now Ianto can understand.