I'm trying to find a story I read a while ago and forgot to bookmark. What I remember is Spike catching sight of Xander at night acting odd. Spike sees Xander hit on Riley, who is horrified, and Xander breaks free and is talking about trying to kill himself. Turns out Xander was scratched by a demon who poisoned him - he will be overwhelmed with lust to the point he will attack and try to rape people before eventually dying horribly, so Xander figures he will just jump to the end.
Spike helps him relieve the lust which helps for a while, and takes him to Angel to see if a cure can be found (or he knows there's a cure and Angel can help, don't remember the details there). I'm pretty sure the sex expands to include not only Angel but Doyle as well.
Can someone help me out with a link? I can't remember the title or author, and I've scanned through all my BtVS bookmarks on delicious and I don't have it saved.