There is no fic (or, fandom being fandom, there probably is) in which Buffy masturbates about no one in particular, gets off, and is satisfied. There is no conflict, no interest for anyone but Buffy. It's fun to visualize, yes, but to write about?
This is how I've felt every time I sat down to write Cuckoocest. There is a scene in Time Enough for Love in which Laz and Lor, who are both clones of Lazarus Long in every respect except for being xx instead of xy, claim that a sex act between the three of them would be masturbation, not incest. I don't think Lazarus ever really believed that claim, despite ultimately giving into the twins' demands. (No man can resist Laz and Lor.)
But the Cuckoos are not only clones of each other (using the term "clone" loosely, as Marvel does; it's unclear how much genetic material they share with each other or with Emma), they are telepathic with each other, often demonstrating a hive mind. Cuckoocest--the Five- or Four- or Three-in-One having sex with itself--is masturbation in a way that normal incest or even clonecest is clearly not.
It's clearly psychoanalytic: Cuckoocest allows for a sexual unification which is at one, quite literally, Self and Other. It is sexual energy directed inward, but at the same time directed at a separate body.
This particular psychoanalytic setup is unique to telepathic 'cest--other than the Cuckoos, the only siblings eligible for this that I can think of off the top of my head are the Witch Mountain kids (either the original or the remake, the latter of which much more explicitly sexualizes the sister)--but the overall setup applies to most 'cest kinks, I think. 'Cesty romance represents an ideal of intimacy assumed to be unreachable by normal romance. Again, I think there is something rather psychoanalytic about that (rather frelled-up) assumption: wives are for lusting after, sisters are for loving, and 'cest is a reunion of the two roles in an overcoming of the whore/madonna dichotomy.
But I like fluffy 'cest. And you might have noticed that I can't write fluff. I enjoy reading it sometimes, but my mind shuts down when I try to write; my main attempts are good fics but not quite in the 'shippy way they were intended to be. I made them work by shifting the focus away from the relationship itself to something else.
There are pairings which, from their canon characterizations, could never be comfortable with their 'cestasticness: Marscest and most brands of Summerscest fit here, I think. But there are also fics where the taboo would be less strongly felt: Laz/Lor (which is basically canon), Annie/Hallie, Cuckoocest, Val/Ender (Val/Peter would be unhappy but for reasons other than the 'cestiness). And while there is much pleasure brought to me by imagining their 'cest, I can't find a way to write about it.
. . .
I notebooked the above passage on the train yesterday. Then I went on to write 200+ words of Annie/Hallie, which I just posted (after making a new offline backup of my LJ, just in case), and about a thousand words of Cuckoocest. The Cuckoocest is PWP; I'm still convinced that it must be PWP, because of the always-already nature of the pairing. No plot is possible. I don't think it is (I hope it isn't) just many many pages of anatomical description; I did my best to capitalize upon the tension between Self and Other discussed above. If someone wants to beta it for me, however, I'd be much obliged; not only because I'm not sure it worked, but also because I'm not a girl, and I think we've well established that femslash is about--perhaps not lesbian sex as such, but--female sexuality in a way that m/m slash is not about male sexuality.