I know that the bible is pretty hardcore on male homosexuals what with the stoning and all. But if you actually read the verses that deal with the punishments for gays it is conspicuously silent on lesbians. so the next time you are talking with a fundamentalist you can just say "But the Lord loves him some girl on girl action"
The first five books of the old testament were first written down in the 8th or 7th centuries BC, during a time in Israelite society when monotheism was developing for the first time and also indivudalism (for instance the extended family disappered in Israel and nuclear family became the norm), when all worship was being centralised in one temple and to one god Yahweh.
The rule in Leviticus (which is a law code) outlawing male-male sex was put in to stamp out the homosexual sex associated with the worship of Astarte the goddess of fertility, sexuality, and war. Since girl-girl sex was not associated with any worship of gods other than Yahweh, they were not mentioned or prohibited.