Hmm.
Perhaps it's possible that humans, as the leading sentient species on the planet, are able to transcend basic instincts sometimes?
These two may well have been taught to suppress and deny any 'evil' impulsive urges, believing that they were the work of the devil or something. Being misinformed doesn't make you evil or stupid or anything.
Perhaps we are becoming so disconnected in the modern world, even the most basic modes of existence are being lost. After all, many need pharmacy medicine to reproduce, not to mention IVF.
Infertility has been around for as long as we have; we just have treatments for it nowadays.[q=Dr Unne]It's neat how most mammals instinctively know how to swim and humans don't, for example. [/q]Babies can swim, then their minds get cluttered with human-related survival stuff.
I would've guessed that a man and a woman could figure out how to make babies without any guidance whatsoever, purely by instinct.
Imagine if you'd had a strict and chaste religious uprbringing, devoid of any sexuality education at all - would you really give in to vague psyhological urges to start intimately interfering with your partner, in defiance of everything you'd been taught?