Some fans argue that we should take Willow at her word - she says she's gay, she's gay. But I've never seen any of those fans acknowledge what Willow actually said. Willow didn't say she was gay - she said "gay now". In Triangle, she was very explicit that her sexuality had changed from what it was in season 3.
I don't know if a person's sexuality can change. Human sexuality is far too complicated for my poor little brain. But if you are going to say what Willow says about herself at face value, then you have to acknowledge that her sexuality did change.