The main conflict in this novel is man vs. society. Genly Ai must adapt to the unfamiliar society on Winter. The people of Winter are very different from the people of Earth. They have no gender and no sexual desire except for once a month. Because gender roles are not a part of their society, the rules are very different than Earth rules.
Genly Ai's attempt to adapt cause his problems. For example, he does not understand Estraven's need to be exiled. He wants to follow Estraven into exile, and struggles because of that. He has trouble understanding the customs and even the language of the people because the lack of gender causes contradictions.
By focusing on a man vs. society conflict, Le Guin is able to point out elements of our own society. She raises the question of nature vs. nurture - are the traits of men and women learned or genetic? Do we put people into boxes because of their gender, thus limiting them? She also brings up the idea of duality - everything that is good can also be bad and vice versa. There are no absolutes - not even gender in this book.