Whatever Spike feels about kissing in gameface, Jack loves it - the thrill of something different, something exciting, something dangerous. He loves Ianto and Ianto's ingenuity certainly has an element of excitement about it, but he's always been drawn to new thrills and Spike in full vampire mode is definitely one of them. He can feel the brute power in Spike's body as he pushes back into the kiss, finally sex and the battle for dominance combined, two of his very favourite things.
He feels the further change that goes through Spike as he tastes blood and it sends a surge of lust through him that must echo Spike's. He cries out into Spike's mouth at the wrench of fisted hair, and then again, bucking hard, when Spike's hand comes down to cup his suddenly achingly hard erection. He presses in urgently, both hands sliding round behind Spike's head again to try and pull him in as he urges, rutting helplessly against him: "Bite me."
There's no sound of anything approaching for the moment. At this point, he doesn't even really care.