Again: sex and gender are *different*.
One is biological, one is social. One your body is born with, one is a product of you and your culture.
"Although biology determines genetically whether a human being is male or female (on the basis of the XX or XY or a variation thereof chromosomes), the state of being neither a man or a woman is
sometimes considered in relation to the individual's gender role in society, gender identity, sexual orientation or any other characteristic. To different cultures or individuals, a third gender or sex may represent an intermediate state between men and women, a state of being both (such as "the spirit of a man in the body of a woman"), the state of being neither (neuter), the ability to cross or swap genders, another category altogether independent of men and women. This last definition is favored by those who argue for a strict interpretation of the "third gender" concept. In any case, all of these characterizations are defining gender and not the sex that biology gives to living beings."
http://en.wikipedia.org/wiki/Third_gender
more information further down, as well as examples.