Well, it looks like the results are in, and I must say, I'm very disappointed.
First of all; why were people assuming that just because Willow or Tara would be allowed to be with another person, that it would automatically relieve the rule wherein they must be together at the end of the story? If they still MUST be together in the end, then you still know what it is you're going to get - a KittenBoard Happy Ending ®. Having someone else in there just spices things up a bit. I don't see it detracting from the board at all, but adding to it.
I'd also like to point out that, given how many kittens are opposed to Willow and/or Tara with someone else, it's unlikely that we would get a flood of fics featuring Willow with, say, Faith (cause we all know she swings both ways...), and give the Willow/Tara relationship five minutes of lip-service at the end.
If we simply instituted a new rule in it's place, wherein you MUST post any and all pairings at the beginning of your fic (which most authors do anyway), I'm pretty sure that would be sufficient to allow anyone who did not want to read Willow or Tara with anyone else at any point - which seems a little extreme to me, but to each their own - to avoid such fics.
As someone who's spent a lot of time in debate forums, I see a lot of logical fallacies in the arguments here - mainly the Slippery Slope Theory. It's the assumption that if we change any small thing, the entire board will go down the drain as far as quality is concerned. If we allow Willow and Tara to dabble in a relationship with someone else, it must then be true that we will have fics wherein Willow and Tara will wind up with someone else in the end, someone else will fill the 'true love' role, or we'll have them paired up in absolutely ridiculous ways, i.e. Willow and Justin Bieber, as watty mentioned. Now, I'm not saying I want to read a Willow/Justin Bieber fic. Joss forbid. I just don't think it's likely that the one tiny change would result in the total implosion of the board, reducing it to fanfiction.net-like quality.
It's like you guys are afraid of it, or something. I'm secure enough in Willow and Tara's relationship to believe that they can take it.
But I suppose none of this really matters anymore anyway. The decision is made, the rules won't change, and we'll not get a chance to throw that particular wrench into the works. I'm disappointed, especially since this seems to have been decided quickly based on knee-jerk reactions (but perhaps that's just my outside opinion on it), but it's nothing to leave the board over...