It has always been an active mystery to me how evolution allowed the male of the species to evolve in just such a way that sexual relations with the female of the species was not only possible but successful. Darwinian theory should make such chance parallel evolution by random chance and natural selection an impossibility. All this is discussed here: Evolutionary Theories On Gender And Sexual Reproduction.

Exactly how did we arrive at two separate genders-each with its own physiology?  If, as evolutionists have argued, there is a materialistic answer for everything, then the question should be answered:  Why sex?  Is sex the product of a historical accident or the product of an intelligent Creator?  The current article reviews some of the current theories for why sexual reproduction exists today.  Yet, as these theories valiantly attempt to explain why sex exists now, they do not explain the origin of sex.  We suggest that there is no naturalistic explanation that can account for the origin and maintenance of sex.

I have asked biologists but no one knows, and there are no theories that even come close to sounding plausible.