Says you.

Says human biology.

But the taboo didn't arise out of nothing.

It arose out of religious morals.

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.

So you would have justified parents fear of homosexuals decades ago?

It may not have been present in our species as a specific reaction long ago

In many places it still isn't and as little as a few decades ago it wasn't.

but the parental instinct to protect has always been in homo sapiens

Agreed.

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.

In this case, merely religious. The only aspect where it is better info is where people realized (not everywhere) we shouldn't force kids to partake in those activities.