A number of prior replies have hit on what I think the answer to this is. While gender is a broad spectrum with all manner of variations, on a practical level we inhabit a society where you're either male or female. And living outside the binary is, well, difficult at the best of times. For many of us, it's a case of looking at the two available boxes and deciding which would be the best fit, if only because it's convenient on a getting-through-the-day-alive-and-happy level. And for some, the opposite gender is a perfect fit and the binary works just as it should.
For someone like me, I'd be happy to be put in the "female" box even though it's not quite a perfect fit, a compromise that's good enough to allow me to live a happy, productive life.
Others have different priorities and goals - some want to fight the gender battles in the trenches, some want to express themselves in greater detail, some want to push boundaries and break barriers, etc., and for those individuals, the gender binary really doesn't work at all. And I, as a trans* person, even though I fit fairly neatly into the binary, will happily smash the binary to pieces if it'll help other trans* people find what they're looking for.
