Are you implying this is instinctual? It isn't.

Says you.

It is a social taboo that plays on the parent's instinct to protect.

But the taboo didn't arise out of nothing. Its roots are genetic, and so instinct is a part of it, yes, though more in a vague general sense than a specific one originally. It may not have been present in our species as a specific reaction long ago, but the parental instinct to protect has always been in homo sapiens, and the specific taboos with regard to children tend to arise with either religious beliefs or better information about those activities. Or, as in this case, both.