I have always been on the mind set that she was in love with Oz. That Oz was her true love. That if the writers didn't concoct some cockamamie reason for why Oz/Willow couldn't be together, (he turns into the thing he hates most) that they would have ended up together.
If Seth Green had stayed on the show, I don't believe Willow would have turned gay. She wasn't gay until he left. She was clearly straight and in deep love with him. I hated this change because I loved Oz, back when he was on the show he was my favorite character. My love of Oz also made me hate Tara, I'm spiteful that way. But more so I believe I didn't like Tara because I hated how she was, how she acted, and how meak and weird she was. Tara was never attractive to me, she was never smart, or funny, or charismatic. She annoyed me, and was the reason (in the show, in my mind) why Oz/Willow weren't together. So I never bought the Willow/Tara relationship.
Then once Tara died, Willow started a small thing with Kennedy... and randomly I loved that, because I liked Kennedy. I bought their relationship more. I still feel, though, that if Seth Green hadn't left the show when he did, they would have ended up getting married and having little wolf babies. So I'm conflicted.
In the end I was always of the belief that Willow was straight until the Seth Green decided to leave the show. Then the writers had to reorganize where her character was going to go, and super witchy lesbian girl manifested. Maybe had she have fallen for a character I liked, I would have been a bigger fan of that relationship, or maybe if just another actor had played Tara, but that didn't happen. So, story wise, Willow was straight, then lesbian, then woulda been with Oz again... so straight, but Oz leaves again... so lesbian again. In the show, because of how long we got to see with her and Tara, it did eventually become believable, but for a long time, for me, it wasn't.
It didn't seem planned out from the start of the show, because it wasn't. Whether you agree with me or think my hatred of Tara has clouded my judgement, don't roast me for my opinion. I just never enjoyed her. I do think Willow eventually became 100% lesbian in the show, so in the end that is all that matters. But it was odd how and why it occurred. If not for the decision of one super epicly cool ginger haired short actor man, Tara would never have been there and Willow wouldn't have liked her.