Sex is real but JKR makes some erroneous assumptions in her attempt to understand it. Genetics is a complex system that interacts with a mother’s womb and body which is another complex system to create the child’s body, brain, and mind which are all complex systems. Boiling it down to the classification of the sex chromosomes is a major simplification that only gets you so far. What sex a person identifies as should be a decisive factor assuming they have the cognitive ability to answer.