Blah blah blah, you guys don't get it.
Oz and Willow had their relationship and it was a good one. It was the first time Willow had ever fallen in love and she was happy. Then, Oz decides to leave town and leave Willow in the dust.
Bam, here comes Tara. Tara is in the Wicca group; something Willow is interested in. These two start doing magic, become good friends, then bam, Willow falls in love with Tara. Once Willow falls in love with Tara, she then realizes that she is gay.
Tara is dead, so here enters Kennedy; Kennedy is gay and so is Willow. Kennedy is much different than Tara is and Willow likes that in her, so they get together.
I look it as Willow fell in love with Tara and then she realized she was gay. I've heard that people who found out they are gay did have feelings for the opposite sex before they found out they were gay; which is one thing that really confuses them.