There's no doubt in his mind that he'll follow through, even if it ends up being a mess of rutting and grinding and nothing but. They need this, both of them, need to get this thing out of their system where it's been chafing for the past six months. It's not even about sex really as it is about wanting Jack spent under him, an uncontrollable bloody mess.
In an effort to get a little closer to that goal, Spike snakes a hand down between them, tugging and pulling at the fastening in Jack's trousers, all the while kissing him. Vampires don't need to breathe. It's becoming increasingly apparent what an incredible advantage that is when you're dealing with mortals.