Hmm...simple meh?
You know, embryologically, the female sex is a more basic, default model. Without the the testis-determining factor on the Y-chromosome, in particular a gene called SRY, the embryo will develop into a female with ovaries. The SRY will induce development of the testis with subsequent production of the male sex hormone testosterone. This hormone is needed for development of the male genitalia. If a mutation results in the loss of the receptor for this hormone, the baby will develop into a girl despite having testes and XY genotype (testicular feminization syndrome).
So one might argue that the female is the more basic, default model and the male sex is a later, "more specialised' development
But I still love the girls..