>>1
Sometimes knowing the difference between the genders isn't as easy you wish they were. Gender is an as fluent definition as sexuality: It's not as binary as you probably think.
We simplify to make our lives easier.
Both genders stem from the same base, we just develop differently.
As a foetus, we're identical up to a certain point, then the basic penis/clitoris structure grows to different sizes for males and females. In the males the area below grows fused to a phallic piece, while as in the females it grows to a hole. There are so very many things that can happen not only during the foetal development, but also after birth during the decade(s) of development, that leaves you with not a stereotype sex development.

To take one very random excerpt:
"An example of the problem with chromosomal definition would be a woman with Complete Androgen Insensitivity Syndrome (CAIS), who would have a 46,XY karyotype, which is typically male. Although she may have been legally registered as female on her birth certificate, been raised as a female her entire life, have engaged in typical heterosexual female relationships, and may even have married before the status of her condition was known, using the chromosomal definition of sex could prevent or annul the marriage of a woman with this condition to a man, and similarly allow her to legally marry another woman. These same issues were faced by the IOC to determine who qualified as a female for the women's competitions.[16]"
( 16: http://www.uksport.gov.uk/assets/File/Generic_Template_Documents/Standards_in_Sport/transsexuals.pdf )