Procreation for a guy is over in a few minutes, but a woman's pregnant for nine months. Same reason as you keep more cows than bulls.
Leave procreation out of this. I think he was referring simply to number of available holes.
Did you hear the one about the prostituite who went to the proctologist ??
She said: "Doc could you make another asshole for me ??"
"Sure.... Why ??"
"Because buisiness is good and I want to open up a second location!"