The Limits of Duality

In her book "Sexing the Body", Anne Fausto-Sterling points out the duality of our perception of the world. We were all raised seeing these dualities and coming to accept them as fact, our young, developing minds unable to see any other way. There are boys and girls, dark and light, good and evil, and the list goes on and on. As we grow up, we continue to categorize things in this fashion, in a Cartesian manner, using our previous experiences and conclusions to draw new ones as we encounter new experiences.
Fausto-Sterling, in a way, breaks out of that Cartesian view of the world. While she is still using logic and reason to draw her conclusions (there's no escaping that), she starts to question the duality that we have been taught growing up. Why do there have to be only two sexes? To better categorize our world (Cartesianism again!!) shouldn't we narrow down such broad generalizations in order to understand each more clearly?
While this next example is maybe less important to our understanding of our world and ourselves, I cannot count how many times you see in film and television examples of the bad guy doing something good. The best example I know is Riddick from the sci-fi movies that share his name. By all of our standards of good and evil he is clearly evil; a murderer and thief, outcast from society. And yet when 'greater evils' come into play, Riddick becomes the good guy, fighting on behave of the same society which cast him out in the first place. In the same way Fausto-Sterling defines 5 sexes instead of just 2, couldn't we also better understand human nature (after all, that is what movies, television, and books try to do) by defining the in-between of good versus evil?
We will never ever part from the use of logic, but we must use it to break down broad generalizations into better defined categories. This will bring about a bit of a paradox, opposing Descartes, and yet Cartesianizing our world even more.