I hated her with Riley and Riley as a person. I couldn't stand him. But yeah, I think her and Spike really would have been interesting. I don't think it woud have worked with Angel because he probably would have lost his soul again.

At that moment in the story, she and Spike having sex under the influence of those ghosts would have been incredibly creepy. Neither of them would have had any genuine interest in the other (as I remember, Spike doesn't realise that he's attracted to Buffy until later in season 4, and even if I'm mis-remembering here, Buffy wouldn't have any interest in him or even know of his feelings for her). It's the same reason I don't like Waiting In The Wings from the Angel spin-off. Characters having sex while under possession or supernatural influence, when they aren't already together or at least obviously mutually interested, is creepy.

She and Spike had enough sex scenes later on in season 6.