I think this article is a very careful examination of the issue, very well written, and I am thrilled by its open-minded viewpoint. I'd like to add, though, that history has not always seen gender as a two-sided coin. The idea of gender dichotomy as "one man, one woman" in our culture has its origins in European and Christian culture. Many cultures do not see it that way. Many Native American tribes believed that there were actually three genders--and when I say gender, I mean gender identity, as opposed to sex, the physical equipment. These Native American tribes had a traditional gender role for a woman, one for a man, and a third, called a berdache in modern historical writings. The berdache was typically a physical man who felt more comfortable in a woman's role. He(she) wore woman's clothing, performed tasks commonly done by women, and married a man.

In India, people born with an indeterminate set of sexual equipment are called Hijra, and rather than trying to be one or the other, they believe themselves to be a third gender with a role in society set aside just for them.

Those are not the only examples. They are the two I am most familiar with, but anyone who is curious about varying world views on gender should check out this wikipedia article: <link>

It is therefore a fallacy to say that human beings are naturally inclined to believe in a two-gender system, a statement which I have heard before. I would just like to finish by saying that while wikipedia's credibility is questioned at every turn, they have a complicated editorial system which guarantees that untrue information does not remain on their website for long.