So Xander was cut (possibly bitten) by a demon during a fight, and he hid it from Giles and the gang because Giles had told him there was no cure. The cut was going to amp up his libido like crazy until he finally ended up raping someone and then dying. Xander obviously didn't want this to happen, so he's off to kill himself when he runs into Spike. Spike becomes very intrigued, finally figures out what's going on, and takes Xan to Angel.
The story ends up Angel/Xander/Spike/Doyle.
Thanks a bajillion.
Oh, and if anyone has any other good demon-induced sex fics, i won't say no. *grins*
EDIT: Woohoo! I knew it would be quick. Thank you white_sin. It's A Matter of Control by Kay