I can't think of any off the top of my head. Though there is The Wanting Seed by Anthony Burgess which is a sci-fi dystopian novel where homosexuality is encouraged as a means of population control. Probably not exactly what you're looking for though. I'll keep racking my brain to see if I can come up with any titles for you!

I've only read the beginning of this book, but I hope this helps: Ash by Malinda Lo. It's a retelling of Cinderella..."However, while structural similarities exist, ideologically Lo's beautiful and dark tale takes the story to a new place. It is not about Ash being found and saved by a charming prince; instead, it is about her courtship with Kaisa, the King's huntress, a relationship that burgeons over time and is based on more than just initial attraction."

http://www.amazon.com/Ash-Malinda-Lo/dp/0316040096/ref=sr_1_fkmr0_1?ie=UTF8&qid=1284105116&sr=1-1-fkmr0

Marian Zimmer Bradley's Darkover series - depends on which book you pick. Awesome, great SF & Fantasy plus strong females.

"Wanderground: Stories of the Hill Women" by Sally Miller Gearhart. It went out of print for a long time but was re-released in 2002 and is available from Amazon.

Isn't the main character in John Varley's Titan(sci-fi) a bisexual woman?