She might have dated women, but she loved two separate boys in her life. Xander first (who she had a crush on for years) and then Oz. They keep referring to her as a lesbian, but she's actually bisexual. I've always felt the possibility was there and that it would be beautifully natural.

I agree with you, Willow should have identified as bisexual not fully gay - it seemed that way how they were writing her initially too. But I don't think bringing Oz back as a love interest would have worked. Neither would a relationship between her and Xander. Tara's death, being at the end of season 6, was too close to the end of the series. If Willow had been given a male love interest afterwards, that'd be her final love interest and it'd have people arguing that her being gay/her relationship was Tara was just a phase. It'd defeat the whole purpose of making a pro-LGBT statement by having a gay character.