I entirely agree with the comments that a lot of things must contribute to the comparative scarcity of f/f. One thought that struck me, reading through this, is that part of the problem may related to the lack of female characters. Not the lack of good female characters, per se. As you justly point out, that doesn't stop us when the not-good character is male. But a lot of slash I've seen (not all, certainly, but a lot) relies on male friendships as a starting point. Friendships between men, and the flipside of the coin, rivalries, make for nice, juicy material. Female friendships, in my memory, are about as rare in the source texts as f/f is in the fan productions. Just another thought to throw in the pot.

That is a good possible explanation for why there isn't more femslash out there (well, in addition to the "most fanfic writers are women, and straight women want to have dicks in their stories" explanation). True, few stories out there focus on f/f relationships, sexual or otherwise. The way many stories are written, the male characters are allowed to exist for their own sakes and/or in interaction with other males, while the females (if they're there) are presented in relation with males. So when fans write stories, the tendency is to either present the females in relation with males (het fic), or leave them out altogether (m/m slash), because there's just little model for female/female interaction presented in the existing story material.

Like, look at LOTR (which I love, mind) - there are very deep, very important relationship between males, but we don't see, say, Eowyn with a female friend she loves as much as Sam loves Frodo or Gimli loves Legolas. We do see her loyal devotion to her uncle Theoden, her hopeless crush on Aragorn, and eventually her love for Faramir - all of them male.

And yes, that probably does say something about the society we live in...