This particular story uses the "fuck or die" scenario. The fic takes places during S4 of ATS and S7 of BTVS. Spike & Xander are kidnapped by Angelus who forces to them to have sex while he watches. Spike is on the bottom and Xander on top and at first Xander refuses because he doesn't want to rape Spike but he says it's okay. The two of them then do it and of course Spike is a pushy bottom.
At the end, Spike tells Xander they won't tell Buffy and then the two of them proceed to walk off even though Spike is walking funny with a grin on his face.