This complicated network of characters is brought together by one strange hypothetical—sexual hermaphroditism. This is a bizarre kind of sexual reproduction that literally does occur in nature on the earth, but on planet Gethen, it is the primary way of procreation. This raises the thematic question of gender, because the plot means that all the characters are literally hermaphroditic and androgynous.
That doesn't mean there is no gender, interestingly, but rather, it frames gender as a kind of role. Each partner accepts a role in the encounter, so that "male" and "female" are more mechanistic, technical concepts, not deeply tied to identity. This effects some aspects of life, but more importantly, the reassignment of gender and gender roles on the planet Gethen actually does not change central aspects of human life.
These characters still attempt to become intimate in serious relationships, and gender is ironically unimportant in that effort. They still make mistakes in their passion, and they still struggle to trust one another. In light of the war that erupts on the planet, and in light of the suffering of the planet's unfortunate concentration camps, these struggles for trust in love reach dramatic heights. The reader will notice that, for a planet with very obvious differences from ours, the essence of the story, and the "role-playing" nature of gender are actually fairly similar to modern life on Earth.
