She still is...
The evil vampire Willow was quite a while ago (don't know which season) but was portrayed even then as being somewhat bisexual.
Anyway, there are a lot of good lines in that show.
From 'The Watchers Guide':
"'I aspire to help my fellow man'. Check. As long as he's not, smelly or dirty or something gross." - Cordelia
"Sorry, I'm an old-fashioned girl. I was raised to believe the men dig up the corpses and the women have the babies." - Buffy
"On behalf of my gender: Hey!" - Xander
"Cavalry's here;cavalry's a frightened guy with a rock, but it's here." - Xander once again.
And some more Willow:
"Even I was bored. And I'm a science nerd." - Talking about a particularly dull biology class.
"I don't get wild. Wild on me equals 'spaz'."
Tom Sinclair
"Subverting Young Minds Since 03/13/2000"