This is an established relationship story; There is a woman (this is going to sound weird) selling...bonds? to demons, and Spike is dressed up in expensive clothes to try to fake-seduce her. And maybe there's an amulet, I don't really remember. Anyway, Xander is jealous; Spike ends up staying the entire night at her place and Xander is convinced he slept with her until Spike comes home the next day. I think. I have one image in my head, and that is of Spike standing in a doorway in linen slacks with bare feet. And Giles says something to Xander to the effect of, "the only thing Spike was looking at was you," or something similar.
Does this sound familiar to anyone? It's been bugging me for days. *g*