I am sorry but I am gonna abuse you for this. Why don’t you then say A white guy should only marry a white woman? Or a nigerian should marry a nigerian? Such retarded thinking is pathetic. Its providing articifial boundaries to a concept that transcends petty ideas of race, color and class.
I certainly hope you don’t bring up children with such a pathetic attitude.
Muzna its no fun being serious. So pervy one liners that may have a different meaning are a lot more fun! 
As for the reason why it happens, why is the sky blue? Why do salmon spawn up stream etc etc. No specific answer.