A Door Into Ocean resembles some of the earliest same-sex utopias in that it's written mainly, though not entirely, from the POV of outsiders, two out of three of whom are male. The premise is that some time in the quite distant past, some female space travelers settled this planet whose surface is nearly entirely water. It's called Shora. (The names in this novel are not very subtle.) The people who live on Shora call themselves Sharers, and they are tall, hairless all over, have webbed fingers and toes, and are purplish-blue.
The Sharers have a very different culture from anyone else, and they are finding attempts to interact with the people from other planets traumatic. They have no hierarchy, no authority, no warfare. Their entire planet is set up to be in perfect balance, and they make all decisions in Gatherings. I know this sounds nauseating, but the author is more than competent at drawing individual characters with conflicts and she doesn't make the Sharers into horrible plaster saints. Their dilemma is whether to consider the others human. Some of this is because some others are male, which is freakish to them, but mainly it's the willingness to hurt others that they think is not human.
She does make the other planet dwellers pathetic patsies an empire ruled by a Patriarch. They have issues with hierarchy, warfare, capitalism, ethnic rivalry, social inequality, and just plain Bad Sex (TM.) You can see how the author was influenced by the world she grew up in--the scenes of the darker purple people facing the lighter non-purple people with complete non-violence are compelling.
This is one of my favorite books, along with its sequel Daughter of Elysium.
The book takes place on the Earthlike planet Valedon and its ocean-covered moon, Shora. The Valans have a socially stratified society which ranges from noble families in shiny urban environments (with the not-so-shiny parts and the poor kept carefully hidden from casual visitors) to dusty little rural towns. Spinel, the son of a stonecutter, lives in one of these little towns, not sure how good his prospects for the future are. Then one day two Sharers from Shora arrive and sit under a tree in the main square which one of the merchants jealously guards as his alone. Problem is, the Sharers don't have quite the same conception of property as the Valans... Soon the Sharers have offered to take Spinel back to Shora as a sort of apprentice/foster son - which his parents think is a great opportunity, since they can't really provide for him if he doesn't get an apprenticeship, and the Sharers are known for their amazing medicines and other valuable trade goods.
It's a major culture clash. The Sharers don't understand deliberate violence, to the point where they don't even have a word for murder - the closest term is "hastening death" - or autocratic control or poverty or many other things Spinel takes for granted. (This doesn't mean that they're all nice and get along, or that they have life perfectly figured out, mind.) Besides that, they live on giant living raft-trees in the ocean, and he can barely swim! He's never had so much seafood in his life, either. At first, the Sharers look low-tech to Spinel, but soon prove to have highly advanced biotechnology. Which it turns out they use to reproduce, since they haven't had men for thousands of years. (This means they aren't sure what to make of Spinel sometimes, but they come to accept him.) And the Sharers are getting fed up with Valedon's trade policies - they get the idea of sharing items so both parties get something they value, and have been trading their seasilk and medicines for metal cables (to tether the starworms which drive the rafts) and gems for a while. Unfortunately, since stone is unknown on their ocean world, some Sharers have become "stonesick" - obsessed with gems. Many Sharers favor closing off all contact with Valedon. Spinel finds himself in the middle of an invasion when Valedon decides it's time to take over Shora to keep the trade going, on their terms, and the Sharers respond with mass non-violent resistance.
Also caught in the middle of this is Berenice of Hyalite House of Valedon, Nisi to the Sharers. Her fiancee is in charge of the invasion, but she was raised mainly on Shora (her family opened trade with the Sharers), and her loyalties are divided.
It's a great story with adventure, epic non-battles, kids learning to grow up, revolution, really neat biotech, and lots of characters struggling with divided loyalties and ethical problems.
Slonczewski has a study guide with some details about Shora's ecology, traditional language and gender polarities and how they're resolved in the book, inspirations for the book in other classic sf (mainly Herbert and Le Guin), and real-world examples of pacifism and non-violent resistance: http://biology.kenyon.edu/slonc/books/adoor_art/adoor_study.htm
Winner of the John W. Campbell Memorial Award for best novel