All I remember is that when Willow did the 'my will be done' spell and turned Xander into a Demon Magnet Spike got to him first. They end up in a hotel and have sex, but Xander is sure that when the spell breaks Spike will leave him. He doesn't but I can't remember what does happen. I think I read this on a LJ but not sure.
Thanks!

FOUND!!
http://spanderfiles.com/arvs/ladycat/wwlw.html

Thank you so much!