The Left Hand of Darkness by Ursula K. Le Guin

This was published in 1969 and was a Joint Winner of the Hugo and Nebula Awards. I promised that I would read all twenty of those joint winners before 2011 was over. I only read eleven of them. Sorry. This is a fantastic book. It takes place on the planet Winter. The humans there have no constant Gender. They spend most of their lives as asexual, non-testosterone/estrogen-impaired people, who therefore are able to work on living in their nonstop Ice World in a cooperative, rational fashion. If you meet someone, and mental sparks fly, then your body follows suit and assumes Male or Female sexuality, while your mate's body assumes the opposite gender. At this point you are "in kemmer." The two of you make a baby. You might be Male one time, and Female ten years later, depending on how the pairings work out. The story here follows an alien, i.e. an off-world regular old Male who is visiting the planet to invite them to join the Galactic Union that he's been sent from. He is ridiculed for being 24/7 Male, and constantly referred to as The Pervert.