Sex and procreation can be found throughout recorded human history, regardless of locale. In mythology (from Zeus boning anything that moved to "be fruitful and multiply" to the Immaculate Conception and numerous creation myths besides), religious teachings regarding sex being for procreation and not mere pleasure, laws governing property rights, inheritance and royal succession, in literature, in plays and songs, in fairy tales and old wives' tales, etc.

For example, here's an article on Samuel Pepys and a sexual manual from 1680 which quite clearly demonstrates that the nature of carnal knowledge was generally known at the time[0]. Along with death and mortal fear, sex and birth comprises one of the archetypal constants of the human experience. It's everywhere. It also happens to be knowledge that's necessary to continue the species, which humans have proven quite good at.

I guarantee that our parents knew how babby was formed, as did their parents, as did their parents, etc. Even our hominid ancestors before the formation of language understood it, because even animals understand it at an instinctive level.

[0]http://theappendix.net/posts/2013/06/this-misterie-of-fuckin...