I personally think it involves the both of them in the hot tub at the Beverly Hilton because Angel won't let them stay anywhere near *his* hotel after the last time, and Spike ducking down under the water to give Xander a blowjob. Only when he comes back up his face is so red from the heat that Xander can't help laughing and laughing at him, so hard he almost slides down and hits his head on the tub.
And Spike gets all pissy at being mocked, or really not, because he loves to hear Xander laugh but he won't say that so he pretends to be mad, and crosses his arms and kicks his feet in the water and won't talk to Xander until Xander grabs him and bends him over the edge of the tub and tongue-fucks him til he can't tell which way is up, which isn't helped by the fact that his head is upside down and he's staring at the wavy pattern on the outside of the fibreglass tub, unless it's real waves and his head's underwater; Spike doesn't know and doesn't really care, especially when Xander starts jerking him off with the same sweet maddening rhythm as his stabbing tongue.
All of which makes Spike a very tired and happy and most importantly pliable vampire, which Xander thinks is good because maybe it'll last the whole three hours till Buffy's wedding starts. Ok, maybe like in the way maybe hell will freeze over and maybe Giles won't go all misty-eyed and pretend there's something on his glasses when he's giving her away, but at least maybe Spike'll be brainfucked and relaxed long enough for Xander to slide him into the tux. And possibly he'll be woozy enough not to throw his entire bag of rice directly at Angel's head -- though Xander's figuring on blowing him in the vestry right before the ceremony, just in case.