The one instruction in Lev. 18 that does specifically refer to women is a prohibition of bestiality.
Excellent point. I'm kicking myself for not pointing it out earlier.
Lev. 18:23 And thou shalt not lie with any beast to defile thyself therewith; neither shall any woman stand before a beast, to lie down thereto; it is perversion.
If the instructions in Leviticus 18 were not specifically directed towards the males, then there would be no reference to a woman in this phrase. Thus the only way to read this chapter that is consistent with the text is to understand that it is addressed to males only, other than the noted exception.
The lack of a similar stipulation with regards to the reference to male homosexuality in Lev. 18:22 indicates clearly that female homosexuality is not included in the list of commands for sexual morality. I conclude from this that lesbianism is not a mortal sin as is sodomy, that the two stand in distinctly different classes of behavior in the eyes of God.