Beyond what the other reply said, there are also examples of societies that have 3 genders - for example, the people of Azad in The Player of Games have male, female, and apex (I think they do have the tech to change gonads, but would not use them because their society is turbo-sexist. Also CW for all sorts of gender stuff in the Culture series, it's NOT all sunshines and rainbows for the people outside the Culture itself).