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).
All the things you point to as tension/interest IMO up until Season 5 read to me as a way of making the main bad guy or new "team-member" more interesting and less 1-dimensional. New characters like that only really gain dimensionality most of the time through interaction with characters we already know and identify with and that's what I think they were doing with Spike and Buffy until Season 5.
I am not really familiar with Superman, so I have no idea how slashy Superman and Lex Luthor are or are not. But there are two things that spring to mind:
1) There's the real reason why Superman/Lex Luthor or Batman/Joker are not portrayed as being attracted to each other in mainstream fiction, and I think both you and Boo Much Fun are very well aware of what it is. It's not because they're mortal enemies. It's because they're both male. And, unlike in fanfiction, mainstream fiction rarely allows the heroes to be gay or bisexual.
Otherwise, how do you explain Batman/Catwoman, James Bond/every female Bond villain ever, Picard/the Borg Queen, Data/the Borg Queen, etc. There's even a Television Trope about it:
Dating Catwoman, or, if it remains in subtext,
Foe Yay.
2) Well, as I said, I am not too familiar with Superman, but do any of the following scenes and lines happen in any of the Superman comics, films, etc.?
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a) Superman and Lex's first confrontation. Lex Luthor (addressing Superman): "Fi, fo, fi, fum. I smell the blood of a nice, strapping young lad." They fight.
b) Superman is transformed into a wimp by a magic spell. Lex nearly kills him, when Superman comes back into his senses and punches Lex: "Honey, I'm home!"
c) Clark/Superman gets a surprise offer of alliance from his nemesis:
Lex: "Hello, handsome!"
Clark: "The world is falling apart and all you care about is that your boyfriend is a big ho? You're pathetic!"
Lex slaps Clark. Superman slaps Lex back.
Superman: "I hate you!"
Lex: "I'm all you've got."
next: Clark has to introduce mom (or dad) to Lex. Mom (or dad) witnesses Clark turning into Superman and is astonished. Lex asks "You mean your mom (dad) doesn't know?!" Superman comes out to his parents. Lex stays in the living room and tries to have a chat with mom (or dad).
d) Some time after: Clark is on a date, he happens to come across Lex with his boyfriend, a loser he knew in high school. It's very awkward.
Lex: "So, let's look at the new boy! I like him... He's got... Vulnerability."
Clark: "And you're with [insert loser boyfriend's name]! Did you lose a bet?"
Clark's date asks him if Lex is his ex. Clark laughs hysterically "no, no, definitely not!"
As he runs out to deal with Lex, they argue some more and Superman taunts Lex about being dumped. Later they fight and Lex taunts Superman about his own failed relationships. "So, that's what it takes to get into Superman's bed?"
e) Later, as Lex is staying/being kept as prisoner/hostage/whatever in Clark's house for whatever reason, Clark's friend mistakenly casts a spell that Clark and Lex should get married. Next thing you know, Lex proposes, Clark accepts, and the two of them spend the next hours constantly making out, whispering sweet nothings, planning their wedding and telling everyone they're madly in love, until the spell is broken.
f) One of Superman's bitterest rivals swaps body with Superman. As Fake!Superman meets Lex and learns who he is, he starts taunting Lex:
Lex: You know why I really hate you, Kent?
Fake!Superman: Because I'm a stuck up tight-ass with no sense of fun?
Lex: Well, yes...
Fake!Superman: Becase I could do anything I want, and instead, I choose to pout and whine and feel the burden of my superpowers? I mean, I could be rich, I could be famous, I could have anything. Anyone. Even you, Lex. I could ride you at a gallop until your legs buckled and your eyes rolled up. I've got muscles you've never even dreamed of. I could squeeze you until you popped like warm champagne, and you'd beg me to hurt you just a little bit more. And you know why I don't? Because it's wrong.
Fake!Superman leaves. Lex is flustered.
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Yeah, I'm sure that nobody would see any UST there. No way.
