This is what I've always thought too. Season 1 & 2 she's in love with Xander. Then in Season 2, 3 & 4 she's in love with Oz. In Season 3 she cheats on Oz with Xander, then fights to get him back. In season 3 she's all hurt that Xander had sex with Faith. In Season 2 she's hurt that Xander would rather kiss Cordelia than her. In Season 4 she's devastated when Oz leaves. Hell, even Vampire Willow from the alternate reality was into guys.
If she were full on lesbian, none of the above would have happened. Oh, she may have dated boys. She may have even mistaken her friendship with Xander as romantic love, as she had nothing to compare it to. I have friends & acquaintances who are gay. They all say the same thing. "They always knew they were gay, but tried to deny it early on because they were taught that it was 'wrong'." (or something along those lines)
No one ever just wakes up one morning and says "I'm gonna be gay now." Which is what Willow's transformation from straight to gay seemed like to me.