It used to be a taboo subject, even between parents and their children.
When sex scenes came on TV, a grown-up would hastily change the channel. If a kid so much as asked about anything related to sex, they would be told to mind their own business instead of talking about "adult only" things.
But times have changed and with that, so have the conversations about sex. The birds and the bees no longer cut it and children no longer believe they come from the sky.