Have wondered about that quite a bit and am not sure what to make of it. Since 2+ seasons without any use of the word "bisexual" is a bit deliberate. Piper seems pretty unambiguously bisexual. ("I like hot girls. I like hot boys. I like hot people. What can I say? I'm shallow.")

I totally get the biphobia in the characters, and distrust of bisexuality in the lesbian community. ("you'll just go back to dating boys eventually"). I'm guilty of that somewhat myself. Not mentioning the word at all (except once, I think) is a bit too deliberate.