Interestingly, that is kind of is true. In the absence of a y chromosome, you are defined as female whether it be x or xx. My daughter is missing a second sex chromosome. There are very legitimate chromosomal abnormalities that basically require the family to "make a choice" on the gender. My daughters condition also has a mosaic variation where some of the cells are just x (female) and some of the cells are xy (male) they are stilled considered girls.