Kat, Dana, Jennifer, Fiona, and Ellynne,
I personally have no problem with Willow being bisexual or being involved in a lesbian relationship with Tara; my problem was (and still is) that how this came to pass was, to quote Oscar Wilde 'badly written'. Like Fiona, I love the way the Buffyverse characters grow and change; so one can look back at the episodes and see what they once were, and how they have become what they are at present.
The problem is that I didn't see anything like that happening regarding this aspect of Willow. For example, there wasn't any evidence that she had fantasies involving or was attracted to members of her own gender, or that she felt 'unlike' other girls. It was, to some extent 'dropped on' fans, unlike other aspects of the characters. Indeed, the fact that it began so soon after Oz's departure could lead people to conclude that the relationship with Tara was to some extent a rebound one, like Buffy's with Riley Finn.
Sadly, I have to conclude that this aspect of Willow was introduced in order to boost the ratings. If not, I feel that a better job would have been made of it.
Murray