There's two facets of sexual behaviour that are unique to humans. The first is we're the only species that sometimes gets aroused on violence or gets violent on arousal. This is an accident of neurology, both feelings a processed close by. The second is we're the only species that cares if anyone's watching. This observation seems to hold across cultures and time periods.

So the answer is, it was always taboo for kids or anyone else to watch. People may not have always been able to hide it effectively, but they were always at least trying.