My DS told his K teacher and the entire class that he saw his parents having sex every morning.

Ya, his poor K teacher, got to take us aside and tell us this, then we got to the bottom of it.
Come to find out, DS thought kissing was sex and since Daddy kissed Mommy goodbye every day well, you see where this is going.
When DS was 4, he piped up from the back seat one day "what is a MotherF'er?" when we got over our shock and asked where he heard that word, "so and so" a fellow 4 year old in his preschool class.
I am old too and I remember when the boys in 2nd/3rd grade would gather on the playground with a contraband copy of
National Geographic.

I don't think this is a new issue.