Society runs on a very basic understanding of sexuality/gender/sex based on physical parts. If you've been paying attention for the past several years you may have noticed that this very black-and-white dichotomy isn't working out so well, especially when the statistically abnormal run up against traditional gender roles. This, I believe, is a place where many voices can be heard. Perhaps you'll lend me yours.
Permit me some musings so I can give a clearer idea of what I believe.
At birth your genes and perhaps some Nurture determines the testosterone and estrogen levels you start out with. Testosterone has been linked positively to what I'm going to call "masculine" behaviors/traits, and estrogen to "feminine" behaviors/traits.
To see what I mean, take a look at the stereotypical Manly Male and his counterpart, the stereotypical Womanly Woman.
The man rises before dawn each day, chops wood for eight hours straight before wrestling bears and eating raw meat to keep his strength up. He pumps iron during this entire process, has never heard of feelings in his life, has constant physical desires, and lives his life by a strict code of honor and logic. He is a loner.
The woman dotes upon those she cherishes, helps any in need and is delicate in nature. She is ephemeral and kind, pure to a fault, and in love with love. She creates a very meaningful sphere and takes care of it, as well as anyone inside. These people close to her are her whole world, and she will fight tooth and nail to keep them safe from any injury. She is nurturing and encouraging, able to see the best in everyone and loves to help that grow. She lives her life by her emotions: passionately, fully. She makes a family wherever she goes.
Few enough are really like either of these ideals. In fact, few are even near it: that's because these represent 100% Masculine and 100% Feminine, respectively. That's rare. Why?
Bell curves. The highest concentration is at the middle, not the edges... but since genders get more of their gender's drug, the bell curve is actually shifted over in the -X direction. Or, to put it another way:
There are more Masculine Men than there are Feminine Men, and for women the reverse is true.
"Opposites attract:" I believe 70/30 seeks 30/70, but people are very different. Hard to say. This model doesn't account for why some people are homosexual and some hetero, but it does explain more.
Society's getting increasingly aware of other sexualities, such as Trans* peoples. In this model, a woman "born into the body of a man" indeed fell victim of a mistake: for one reason or another, too much estrogen and not enough testosterone was given, creating feminine behaviors/traits in a body more comfortable with the masculine.
I believe this model gives a much more flexible and natural heuristic to work with. By all means, tear it apart. Let me know what you think of it.
My apologies for not citing my sources.