They're bi sexual only if you want them to be,the writers have given you just enough to satisfy a fantasy but they're not committed to actually having them identified as such. Catwoman was a case of the writer merely testing the waters, she never followed through on it, we never got a story exploring that side of Catwoman not with Eiko or some other character.
Same goes for Ivy,she's only bi to satisfy the fantasies of a shipper crowd and serve another characters story. When that side of her gets explored somewhere else and for herthen I'll personally consider her a successful bi. Right now it's just Harley and her fantasy and the fantasy of a vicious Harley fan club.