Although many people think of previous decades as being repressed, at least one sex-ed book from back in the day was pretty graphic when it came to depicting sex. It's called How Babies Are Made, and was published by Time Life in 1968. It starts off using plants to describe sex:
The seed falls onto the ground. With the help of sun and rain, it will grow into another plant.
That's not too out of the ordinary. So far so good. It then starts depicting animals. First birds:
To send the sperm into the hen's body, the rooster climbs onto her back and places his opening against hers. Then his sperm move into the opening in her body.