Well, the very simple version is that there's a whole bunch of people around that consider themselves both male and female. And, more importantly, that these two genders don't cancel each other out, but exist independently and without conflict.

Put simply, by defining male and female as opposing ends of a line you make the explicit assumption that they're polar opposites, whereas the evidence I have seen seems to indicate that it's perfectly acceptable to say, have a man who is totally masculine and totally butch, with a fully active femininity. Nothing, I think, about male or female naturally opposes each other at all.

And the reason that I think that this breaks the genderline is because even if you say that a male and female dominant person falls straight in the middle of the spectrum, you've just wiped out the natural position of a person who doesn't cleave to either gender.

I think that at the very least male and female are two axes, not two poles. I'm also fairly certain that there could be other axes that exist, but of course the ones that I think of most people wouldn't exactly consider gender-related (although I think they are to many people's genders - things like sexual identities and BDSM identities).