Ursula Le Guin paints a world in her book "The Left Hand of Darkness", that is almost impossible to imagine, and that's what makes it so interesting. Le Guin's alien world of Gethen is one of perpetual winter, filled with a people much like our own. The primary difference between our species and theirs is that they are all of a neutral sex, with the exception of a few days a month, where they decide which sex they want to be. This difference seems silly, and almost impossible to us, for the duality of male and female is so ingrained in our lives.
When the main alien character, Estraven, is first seen, it is in the context of a large parade with the nation of Karhide's king. A "king" that is without, and with no desire for, a queen. Estraven serves as the sort of "right hand man" of the king, giving advice to Genly, the male protagonist earthling. It is virtually impossible for us to imagine these characters initially as anything but male. There is absolutely nothing resembling sexual tension between Genly and Estraven in their early interactions, and it seems no different than any interaction between two males. The social positions occupied by many of the characters are those that we would typically associate with male, such as a "king", and so we have to constantly remind ourselves that the characters are actually gender-neutral.
Le Guin's use of masculine pronouns for all the characters makes it even harder to break out of the normal male image. This appears to be an issue Le Guin was trying to avoid, as a future short story called "The Winter's King" about the same world was published with both all male, and later with all female pronouns. However, the decision to use "he", no matter its intentions, again forces the reader to constantly remind themselves of the true gender neutrality of the aliens.
Psychologically it is easy to understand why it would be so hard for people to grasp this concept. If I asked you to think of images of ten different women, and then of ten different men, it would not be a difficult task. However, if I were to ask you to picture ten different gender-neutral people, it would not be easy at all. The concept of being both and neither gender at the same time is like nothing we encounter in our everyday lives. Le Guin challenges us to imagine an alien race that is just like us, and yet far from us at the same time.
As if it wasn't hard enough to imagine how the humanoid aliens looked, their lack of gender comes along with a lack of sexual drive. Le Guin seems to tie this lack of sex drive to other properties of the culture, such as its slow technological advancement, and its utter lack of wars, of even the word war. Lets be honest, no sex drive, technology rush or war? Sounds like a pretty boring world. The inhabitants of Gethen lack the motivation to improve their technology past what they believe they need, as shown with their centuries old heater design, which could easily be improved upon, but it is felt that it is good enough. Video games will never be as good as they could be, and cars will never be as energy efficient as possible. The satiety with life is a concept that humans cannot relate to.
The world of Gethen is similar to ours on the surface, but radically different underneath it. Le Guin challenges our imaginations like no other author I have ever read, and sheds light on just how ingrained some concepts, such as sex, are.