Blue Lagoon, The
The original Blue Lagoon, starring Brooke Shields as Emmeline Lestrange and Christopher Atkins as Richard Lestrange, was released in 1980 (Return to the Blue Lagoon was released in 1991). The movie was the 9th largest money maker of 1980.
In this movie, two young cousins (Emmeline and Christopher) are stranded on a lush tropical island after a shipwreck, along with the ship’s cook. The cook forbids them from going to the opposite side of the island, where he had once seen evidence of native human sacrifice. After the cook passes away, the two make due on their own, growing up together and living on their resourcefulness. As the years pass, the cousins begin to fall in love with each other, but with no connection to the outside world, they do not have the maturity or knowledge to know how to deal with their hormonal situation.
Eventually, human nature takes it course, and Emmeline becomes pregnant. Neither understands the concept of pregnancy nor realizes a baby is on the way, but when the baby boy is born, they naturally take to raising him. At the end of the movie, the family ends up a drift in a boat as it is swept away by the tide, and they are rescued by their original family, who had been looking for them for years.