The natives of the planet Gethen are androgynous, neither male nor female while also being both at the same time. This aspect of their biology has put its mark on every aspect of their society, from politics to mating rituals (obviously) to social mores. Seems simple enough until you actually have to live in their society as Genly Ai does. Ai can't figure out how the Gethenians see the world because they are so alien to him. Not just because they are aliens but because he views his world as a man. In trying to force Gethenians into his idea of gender roles, Ai only suffers massive culture shock. Obviously this aspect of Ai's character will have to change if he is to succeed on Gethen. But, easier said than done. The Left Hand of Darkness asks us to consider how deeply ingrained our ideas about gender are.
Although the Gethenians are biologically androgynous, they act like men in every other way.
To say that Gethenians "act like men" is actually a more sexist position than the ones critics accuse Le Guin of, because it assumes that men always act in a certain way.