The Birthday of the World, by Ursula K. Le Guin
But just to help keep the titles with the stories, this is a collection of mostly Ekumen short stories. There are a couple of beauties about relationships on the planet O, where people are divided into moieties - Morning and Evening - which is passed to the children from the mother. A marriage involves 4 people; 2 male, 2 female, one of each from each moiety, and the opposite-sex couple with the same moiety aren't allowed to have sex. This seems like a somewhat unlikely form to evolve, but humans are notoriously clever at getting themselves into weird predicaments, and it gives rise to all sorts of interesting thought-provoking questions. If moiety is passed down from the mother and you cannot have sex with someone of the same moiety, then it takes the more usual restriction against inbreeding to an extra degree; not only can't you breed with any of your immediate ancestors, but you can never breed with anyone whose mitochondrial DNA was ever mixed with yours; it creates two completely separate streams, trading places down through time like dancers in a hay or strands of rope. I like the image, and I'm not entirely certain of the genetic effects - any mutation in that DNA, for instance, would have to be immediately dominant to survive, since it could never cross-breed with itself. And then there's the social implications of the fact that, depending on how restrictive society is about children out of wedlock, you essentially cannot breed in their society without being bisexual. Which brings you back round to the debate about how much of sexual orientation is nature, and how much culture.
Fun stuff to think about. And the others are just as good. This is the book that I was inspired to finally read by the decision that Ursula K. Le Guin is the person in the world I would most like to have dinner with (besides my lovely wife of course, but she'll forgive this minor infidelity I think.) And I enjoyed it so thoroughly that I actually sent her the invite to dinner, which letter I suspect I will be embarassedly amused by for the rest of my life. She is gold; read it all.