I could see all of that but it was poorly explained. The show just made it seem like Willow fell in love with Tara but retained her attraction to Oz and Giles in S4 and then in mid-S5 just woke up gay, making jokes about the penis-monster like, "If I wasn't gay *before*" in sharp contrast to spurty knowledge-type metaphors finding their way into her conversations.
Willow's change from still being attracted to guys to being gay was way poorly explained. Also, I don't know if it's the WB's censors or AH's discomfort with playing a lesbian or AB seeming very dour and non-sexual but I don't get this, "It's obvious that Willow felt a physical attraction with Tara that she never did with Oz and Xander." Still even with that criticism of the story put aside, I agree with Dagenspear that Vamp Willow being fully in command of her sexuality and being completely bisexual throws a spanner into the "Willow is gay" argument. I've met Vamp Willow. Vamp Willow is, well not friend, but appreciated titillating TV character of mine. If Vamp Willow wasn't being fully sexually satisfied, she'd drop Vamp Xander as a mate or her sadistic-sexual games with Puppy!Angel and find a girl to do it with.