In the world Le Guin created everything is informed by a principle of duality. There are complementing nations of Karhide and Orgoreyn sharing a continent, both male and female are residing in each individual, there are even lines of a poem evidencing it: "Light is the left hand of darkness and darkness the right hand of light. Two are one, life and death, lying together like lovers in kemmer, like hands joined together, like the end and the way." This principle is disturbed by Genly Ai.
Ai is an odd thing in a world where everything has a pair. His journey with Estraven over the Ice at first seems to finally integrate him in the strange world and make another duality example, but it can't be so. If we look at Ai as one half (male) we must look at Estraven as a whole (male and female) and if we see Ai as a complete person we must see Estraven as two. It is evident that perfect, balanced duality between the two cannot be accomplished regardless of the love and friendship they have grown. Estraven is aware of that and, being a distraction for Ai who is in constant kemmer, decides to let himself be killed to ensure Ai can complete his mission.
Arrival of the Star Ship marked not just the completion of the mission but restoration of the principle of duality. Ai wasn't a lone stranger anymore. There were now twelve of them, men and women. Could we assume there were six of each sex? It certainly seems plausible. But just as everything was set in balance Ai disturbed it again, seeing his own people as strangers and taking comfort with Gethenians.
In the world Le Guin created everything is informed by a principle of duality, and so is the existence of the principle itself.