The facts on the ground, whether we like 'em or not: Buffy had sex with a woman of her own very free choice (twice), she liked it very much (both times), and over the course of the comic she has displayed every quality toward Satsu, physical and emotional, that we commonly consider to add up to "love." More, in fact, than she has displayed with some of her "official" lovers in the past.
Um, you make it out to be a much more meaningful relationship than Joss thinks it is... And he kinda, you know, wrote it...
But my point is that characters change over time. Willow did, and so did Buffy. I don't see one case as being more out of character or unbelievable than the other.
Again, you're defending it more than Joss would. According to Joss he intended to make either Willow or Xander gay from day one. Then
The Wish was one of the first big indicators that she had that tendency. And "Dopplegangland" had Willow saying that her AU-self seemed "kinda gay."
So with Willow we have indicators a full year before she came out, with claims of Joss planning it as much as three and a half years earlier. With Buffy the first indicator we had that she was gay was when she had sex with a woman. How is 1-3.5yrs = 0 seconds?
I can think of only two times when Buffy (the character) even addressed homosexuality, and she certainly didn't indicate that she had tendencies:
"The Yoko Factor" (S4)
Buffy: Judgmental? If I was anymore open-minded about the choices you two make my whole brain would fall out!
(in response to Xander dating Anya & Willow dating Tara)
"Once More With Feeling" (S6)
XANDER: I bet they're not even working.
BUFFY: Who now?
XANDER: Willow and Tara. You see the way they were with each other? The get-a-roominess to them? I bet they're- (glances at Dawn, catches himself) Singing. They're probably singing right now.
GILES: I'm sure Willow and Tara are making every effort.
XANDER: Oh, yeah.
BUFFY: Xander.
DAWN: Buffy, it's okay. I do know about this stuff. Besides, it's all kinda romantic.
BUFFY: No it's not!
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And that's all I really have to say about it. I didn't see any build up in the 9-ish years we saw Buffy that she would even consider a lesbian relationship. That's the only point I was trying to make.