In the afterword of the 25th anniversary edition of the novel, she stated that "The Left Hand of Darkness is haunted and bedeviled by the gender of its pronouns", and that she no longer believed that the masculine pronoun in English is generic, as she had when she wrote the book.
From the Wiki-Article about the Book
she used masculine pronouns as generic pronouns, which is still often done.
I guess it's a product of it's time in that regard, simmiliar to Tolkiens use of "Men" in Lotr being read as "Humans" in many cases when it was first published.