It does send a thrill through him, knowing it's him who's made Ianto like this, so open and needy and so very, very thoroughly debauched. He strokes Ianto's back soothingly with his free hand as he feels Ianto start to open up around his fingers, crooning, "Don't worry, sweetheart, I'm going to give you what you want. What we both want."
Because he does want it, desperately, wants to be deep inside him, sharing that heat with him, feeling everything it does to him, to get fucked when he's like this. One last stretch with his fingers, and then he pulls them out, grabs the lube again to slick up his cock, and then he's stepping in between Ianto's spread legs again, and teasing his cock over that heated skin, watching the little trail of lube and pre-come it leaves behind until finally it reaches its destination and he starts to push inside, letting out a breath he hadn't realised he'd been holding.