And yes, I know she tells herself she was only sleeping with Spike to feel something, anything, but surely Buffy could have found someone a bit more reasonable to scratch that particular itch if that's what she desired. Not a huge quibble with the season, but still.
There aren't very many men around who could scratch that particular itch and were as safe as Spike. She could have done a four-way with the Trio, I guess, but she didn't know who they were or how to contact them for most of the season.
The problem is that she wasn't looking for mere sex, she was looking for sex with someone she disliked, a villain who she should be fighting.
Spike was safe in that regard. She could be relatively certain that he wouldn't kill her in a moment of weakness and the chip meant that he also wouldn't be able to kill anyone else.
If she had picked someone else she would have picked a monster of the week most likely and that wouldn't have turned out very well.
For me, their verbal jabs was no more an indication that Buffy and Spike are attracted to each other than when Superman and Lex Luthor trade jabs and I hardly think there's some unrequited attraction there. (Good lord I hope not).
There is a lot a UST between Lex and Superman, especially on Lex's side. It varies from writer to writer and story to story, but you can see it in most modern depictions. In Smallville you can cut the homoeroticism with a knife. In the DCAU it's mostly subtle but it becomes quite obvious in some scenes. In the mainstream DCU Lex went so far as to have a kid with Superman.
I'm fairly certain that Clark could have saved both himself and Metropolis a lot of grief if he just bent Lex over a desk.