It's not something he expected to hear and surprise - very pleasant surprise - goes through him, displayed by a slowly widening smile, followed by a low chuckle.
"Remind me to ask you about that again some time when you're not still high on endorphins," he says fondly, because there's no way he's going to hold Ianto to anything he comes out with when he's like this, but it's very definitely something he will bring up again at some point when Ianto's more lucid.
And then, because high or not it's still a massive and rather wonderful development, he leans down and pulls Ianto into warm, delighted kiss.