The International Olympic Committee (IOC) Medical Commission now recognizes that the sex hormone levels are a much more relevant factor in determining sex/gender for sport's competitions, than the sex chromosomes - even this is debatable.
What is the biological function of the two sexes? Why, it's to make the next generation via sexual reproduction.
And only XX individuals can get pregnant and give birth.
And only individuals with at least one Y chromosome can produce viable sperm and impregnate someone.
One of the fundamentals of biology is this: the phenotype comes from the genotype.
And in this case, the genotype is sex chromosomes and the phenotype is tits & pussy or cock and balls.