Current documentation on this issue says that is one of the great puzzles of biology...your explanation is overly simplistic, and does not explain anything. Sorry.
http://content.karger.com/ProdukteDB/produkte.asp?Aktion=JournalHome&ProduktNr=231547
Sexual Development:
Aims and Scope
Recent experimental and clinical research have led to impressive advances in our knowledge of the genetic and environmental mechanisms governing sex determination and differentiation, their evolution, as well as the mutations or endocrine and metabolic abnormalities that interfere with normal gonadal development.
The new journal ‘Sexual Development’ aspires to provide a forum for this rapidly expanding field. Its broad scope will cover all aspects of genetics, molecular biology, embryology, endocrinology, evolution and pathology of sex determination and differentiation in humans and animals. It will publish high-quality original research manuscripts, review articles, short reports, case reports and commentaries. ‘Sexual Development’ is a modern journal managed by an internationally renowned and multidisciplinary editorial team of three chief editors and ten prominent scientists serving as section editors, supported by a distinguished panel of editorial board members. They are committed to ensuring fast and author-friendly editorial processing and peer reviewing. Contributions from the scientific community are invited to make ‘Sexual Development’ the long-awaited and viable forum for basic and medical research on sex determination and differentiation.