After reading the answers already given, I think there are some points and some exegetical contexts that have been over looked or misrepresented. To answer the original question "So is it fair to say that female homosexuality is not prohibited by the law of Moses?", I would say Yes and No. Yes in that scripture does not prohibit all female/female relations, but No in that female/female only relations ARE prohibited. Allow me to expand:
It was said earlier in an answer that when scripture says “man” that it is referring to all mankind including women and citing Genesis 9:6 as a prooftext. However, this is untrue. But, let us examine it to see why.
Genesis 9:6 The Scriptures 1998+ (6) “Whoever sheds man’s [H120] blood, by man his blood is shed, for in the image of Elohim has He made man.
From H119; ruddy, that is, a human being (an individual or the species, mankind, etc.): - X another, + hypocrite, + common sort, X low, man (mean, of low degree), person.
Here we can see in the original Hebrew that the word translated into the English is a word meaning a human being as a part of all humanity. So the standard English translation here is fairly ambiguous without knowing the meaning of the original Hebrew. Now let us take a look at another verse..
Leviticus 18:22 KJV Thou shalt not lie with mankind, as with womankind: it is abomination.
Here a clear distinction can easily be made from the English translation, but we can only make the distinction due to the fact that both terms “mankind” and “womankind” are used in the same verse. However, if only the word “mankind” were used here in the English translation then the reader could easily be made to think that “mankind” was referring to all of humanity if the reader did not understand the original Hebrew. Fortunately the original texts use the words referring specifically to males only first and then to females only. Neither word is referring to the human race as a whole.
Now what can we understand from this verse as relating to the question at hand? Here we get the clear command that men are not to have sexual relations with other men as they do with women. However, there is never such a command given in either the Torah or the entire Tanakh (aka Old Testament). There are sexual prohibitions for women, but none relate to women having sexual relations with women. For instance, a woman is prohibited from having sexual relations with an animal.
Leviticus 18:23 The Scriptures 1998+ (23) ‘And do not have intercourse with any beast, to defile yourself with it. And a woman does not stand before a beast to mate with it, it is a perversion.
Here the text is sure to include both men and women, specifically in fact, in the prohibition of beastiality. But we never see such a prohibition against women have relations with women as we do with the prohibition of men and men or humans and animals.
The Messianic Scriptures, Brit HaDasha, or New Testament goes even further in clarification on this issue.
Romans 1:26-27 The Scriptures 1998+ (26) Because of this Elohim gave them over to degrading passions. For even their women exchanged natural relations for what is against nature, (27) and likewise, the men also, having left natural relations with woman, burned in their lust for one another, men with men committing indecency, and receiving back the reward which was due for their straying.
From these verses we can see that men left the natural use of a woman and committed indecency with other men (i.e. sexual relations). Similarly we see that women left the natural use of men for unnatural relations. Notice here the distinctions though. Scripture clearly makes it a point to say in the case of the men that it was “men with men” and that they were “committing indecency”. In contrast, when speaking about the women, all scripture states is that “women exchanged natural relations for what is against nature”.
As proven earlier, we know that scripture up until this point never prohibited women being with women. We also know that scripture does not contradict itself. So how do we reconcile the passages from the Tanakh (aka Old Testament) and the Messianic Scriptures (aka New Testament)? It is actually rather straight forward when one takes the context of the entire Bible into account with what all of scripture says about men/husbands and women/wives. We know that men are to be the heads/leaders of their household (Genesis 3:16, 1 Corinthians 11:3) and that wives are to submit to their husbands (Ephesians 5:22, Colossians 3:18). So natural relations for a woman would be under the headship of their husband, submitted to their husband, being a helpmate to their husband, and helping to reproduce. To leave natural relations would be to completely leave the man out of the equation altogether.
But how does a woman do such? Currently, in this day and age, it would be what we call “lesbianism”. Lesbians only go for women with women. In such a relationship, no reproduction is possible (which is unnatural), there is no husband in the relationship so there is no head (which is unnatural), and there is no husband to submit to (which is unnatural). So woman/woman only (lesbianism) is anti-scriptural as it is completely unnatural and circumvents the commands of scripture (headship, reproduction, submission, etc).
However, this is where we get into the other part of the answer. Given everything we have learned so far, scripture does not prohibit bisexuality for women. As long as a woman is submitting to her husband as her head/leader, being a helpmate to him, and helping him with reproduction of the human race (be fruitful and multiply), then if that same woman has relations with another woman, then she is not breaking scripture. There is nothing in scripture to prohibit such a situation.