It's fine that this is somewhat one-sided in the dick fondling department, because Spike still thinks he's getting the better end of the bargain. It's been awhile since he got to eat his fill and Jack... Jack is special. Not quite as potent as a Slayer but not far from it, blood delicious enough to give him a bit of a high.
He tightens his grip around Jack's back when he comes, making an effort to keep him steady and ease him into the darkness. If you have to go, this is a pretty good way to do it and he has no doubt that Jack will say the same when he comes to in a couple of minutes.