Willow was originally straight
Willow doted on Oz when they were together. They had a monogamous, heterosexual relationship, except for Willow's brief fling with Xander, which she hugely regretted because she hurt Oz so much. After Oz left Willow she was utterly crushed and not looking for a lover.
Willow realises that she can have a relationship with a woman
She starts a friendship with Tara because they both practice magic. This progresses for several episodes until Tara shows that she has a romantic interest in Willow. At this point it seems as though Willow realises she can be attracted to women and thinks "Yeah! Why not have a relationship with Tara?" and reciprocates. Willow doesn't seem to be aware that she can be attracted to women until this moment.
Willow is now a monogamous lesbian, not bisexual
Willow reassures Tara several times during their relationship that she is gay, she is a lesbian and she is not attracted to men. How did her relationship with Tara change her sexual orientation, instead of expanding it? Or, why is Willow now a lesbian instead of being attracted to both women and men? She can still be monogamous if she is bisexual. It doesn't mean that she changes between gay and straight each time. I'm looking for an in-universe explanation because I guess that a bisexual character would have been even more controversial than a lesbian character at the time.
- Straight/heterosexual: Only attracted to people of the opposite sex
- Lesbian: A woman who is sexually attracted to women but not sexually attracted to men
- Gay/homosexual: Someone who is only attracted to people of the same sex
- Bisexual: A man or a woman who is attracted to both men and women
- Monogamous: Having no more than one romantic interest at a time
When I am in a relationship I still find some people attractive, I just choose not to act on it because I only have monogamous relationships. Although I am heterosexual, if I was bisexual I would still be attracted to both other men and women, even though my partner would be only one of those.