You know, for something you've labelled PWP, there's an awful lot of subtext here. This is a perfect example of what I was talking about earlier in my fanfic post -- with some exquisitely chosen phrasing you've balanced this fic on the razor's edge of an unhealthy relationship and one that has a genuine chance. You've taken wonderful advantage of the fact that we know the history already to give us an amazing glimpse into what could be possible between these characters.

Plus you reference Alan Moore.

Plus it's HOT. Xander leaning over and licking Spike's spine?????? Gah.

My favourite line?

There comes a point when irony cuts like a knife.

My only question:

He runs his callous hands over the smooth ripples of Spike’s ribs and abs

Do you mean callused? Even though callous hands gives another take on the scene ... but not one I think you meant.