At least Spike had the opportunity to ruminate with his thoughts and get all hot and bothered.  Yeah.. Love the hot scenario you laid out.  Did the trick, LOL!

“You’ll do.”  Oh boy that was cold.  Harmony should rip his head off. 
 

Oh thats cold. I do kinda feel sorry for Harmony. She was clueless in life and I think it is actually worse now that she is a vamp.

Buffy has his head all screwed up and he has no idea what to do now.  At this point in time Spike is willing to take what he can get. He doesn't realize that he is in love with Buffy, and I think he had feelings for her long before he ever even admitted it to himself.  He just doesn't want to admit them, because that means his whole life gets turned around. He can't be the big bad anymore and be in love with her. He has to be a good guy and bothers him. He killed two slayers, had a damn good rep as a vamp and now one girl, one slayer has killed it all. First he never killed her, she beat him every time. Put him in a wheelchair, brought Angelus out and by default Dru left him. He lost the Gem of Amara, got chipped. Got engaged to the slayer because of magic. Yeah I can understand why he feels the way he does. He went from being feared to being harmless and tame and housebroken. Gotta suck.