Oh, what Heck. I'll do it. I'll take the other side of the Willow and Oz as a future couple argument, because contrary to the way it appears so far there is another side.
First of all let's address the "what the fans will accept" argument. It suggests there is something so deeply ingrained in the text, so clearly spelled out, so unmistakable, and incontrovertible that any possible, future developments can only be portrayed one way.
I'm not sure that's the case. I suggest what's more possible is this "what the fans will accept thing" is more a case of the louder voice insisting we can only look at Willow as being lesbian and only lesbian with no possibility of bisexuality. I'm thinking maybe there are lot of fans who are less locked into this, than they are content to simply let the louder speak for them.
You'll see this kind of thing happen on boards where the louder voice is decrying the evils of filesharing, or global warming, or some issue where the media will only allow one side to be heard. People are first saying stuff like "everybody knows, or it's clear, or it can only be", then someone steps forward, and says "wait a minute, maybe things aren't so clear". All of a sudden it turns out there were a lot of people having the same quiet thoughts. It's not unusual to discover they were the silent majority.
So Willow and Oz...could it happen? I say, why not?
Let's discuss this point.
You can't go back on who you are and I know Joss won't do that either, because he 'gets' people/characters.
I wish I had that kind of penetrating wisdom to allow me to look into the mind of Joss, and pull out only what he can and cannot, or will or will not do.He surprises me all the time with the things he does. When he made Willow a lesbian in the first place that surprised me.
I saw her as being genuinely attracted to Oz.
I was under the impression the physical side of their relationship was more than satisfactory. When Willow suddenly felt a physical attraction to Xander I couldn't help thinking that attraction was, you know...physical.
Then she was a lesbian. She was hetero, very much hetero, then she wasn't. That's the kind of thing Joss will do. I'm not saying he will make Willow bisexual. I'm saying if he wanted to make Willow bisexual, he not only would, but he could make us believe in it.
But back to the text. Then there was Willow and Tara. From time to time even during the Tara era we would get the impression Willow was sparking on the guys. Tara would get the angry eyes, and Willow would instantly excuse the impulse away. At one point Willow accuses Tara of harboring doubts as to her (Willow's) commitment to her gay sexuality. Tara doesn't back away from the insinuation. One gets the feeling she really does have these doubts. Tara was a pretty good judge of Willow's flaws. She was the first to notice Willow's addiction to the magic. So why can she only be right about the magic, but she must be wrong about Willow's buried attraction to guys?
Then there's this idea that there's such purity in Willow's character it makes it impossible for her flip flop from one commitment into another. To that I simply say, huh? Did you watch the show? Willow's all about the flip flop. When we first meet her she's this pure-hearted virgin, afraid to take chances. Suddenly she's leaving a nightclub with the first sleazy come-on artist who shows her attention. Yes she did it at what she thought was Buffy's encouragement, but the point is she did it. Then she's committed to Oz, but she's breaking that commitment to check out Xander. Oz finally leaves her. She's distraught. Suddenly she's a lesbian. She's committed to her education, but no wait then there's this magic stuff. Let's stay in Sunnydale and do that instead. Tara is the love of her life. Tara dies. This hot young thing turns up and shows her some attention. "Hey, let's try that".
There's nothing about Willow's character which demands she can never go from hetero to gay to bi.
In Dopplegangland Willow expresses concern that her vampire self was a little bit gay. Following that there's a little interplay between Buffy and Angel which suggests vampires can take core bits of their sexual identity with them in the transition from human to vamp. Was I the only one who noticed Vamp Willow was bisexual?
And if Willow is bi, why not Oz? Remember this bit?
OZ: It was stupid to think that you'd just be... waiting.
WILLOW: I was waiting. I feel like some part of me will always be waiting
for you. Like if I'm old and blue-haired, and I turn the corner in Istanbul
and there you are, I won't be surprised. Because... you're with me, you know?
OZ: I know. (Pause) But now is not that time, I guess.