I figure the most likely explanation stems from monkey brain going, "Grunt. Lady have baby. Lady and baby not die. Lady have more baby. Have baby good. Grunt, grunt." Some of us just do...a lot more with it.
That kind of relates to another possible explanation I heard once. I'm no psychologist (fuck if I can figure out my own brain, let alone anyone else's) but the theory goes something like, "As with every living being, we're wired on the genetic level to make more of us. But what's supposed to happen is that when pregnancy happens, the drive to mate diminishes and the drive to provide increases, since it's not like you can make a baby with something that's already pregnant, and that's where the drive to have sex comes from. A pregnancy fetish results from the absence of that change." Of course, humans are one of the few creatures that have sex specifically for pleasure (and, as has been pointed out by those more cynical than I am, specifically to make our partners feel like shit), so something's off there.
But in all seriousness?
It was probably Groggok. That prehistoric pussy fiend would fuck anything with a pulse, so it's only natural he'd pass on his sex drive. It's probably his fault we have furries and foot fetishists, too.
Pic related.