A conversation via PM brought to mind an interesting thought, if two people could grow up together with no outside influence shaping their sexual viewpoint, what would happen when they came of age?
Now having such a thing happen would be all but impossible under normal circumstances. Children are raised by parents, aunts and uncles, cousins and neighbors. Both by means of observation and by edict from the elders in their lives, children are influenced in their sexuality from an early age. Much of this comes down to protection. In our world not even the smallest of children are safe from some threat of sexual predation.
Roughly 100 years ago there was a novel written about this very subject and it was made into a motion picture in 1980 titled The Blue Lagoon. In a nutshell, it was about two small children marooned on a tropical island. They matured and eventually nature did indeed "take its course". The movie was somewhat controversial in its time. Brooke Shields was 14 years of age when it was filmed and the state of dress (more accurately, undress) portrayed in the movie sparked some criticism that a minor was being exploited. (Brooke Shields has since stated that body doubles were used.) Anyhow, I think that the story line of the movie is an interesting thought experiment regarding the development of human sexuality. In the story, they discover love making on their own and don't know anything about pregnancy. They are completely surprised by the entire experience but raise a child in spite of their ignorance.
One can not help but wonder what would really happen on such a situation. There is no doubt in my mind that two people in such a situation would discover intercourse. There is also little doubt that they would probably produce children in rapid order, it's only natural. I suspect that a theory of fertilization would occur to them, especially once one of the children started to bear a strong resemblance to the father. Perhaps some method of family planning would occur to them; coitus interruptus being the most obvious. Discovering the fertility cycle would probably not happen unless they lived, and were fertile, for a very long time indeed. Such discoveries come about much more readily if there's an experimental group of some size to observe.
The obvious question, within the context of this site, is whether they would discover a gentle form of lovemaking. I think it's impossible to make any meaningful statement regarding this because none of us know what it would be like to be raised without any outside influence upon our sexuality. Actually, unless infants were raised by (gender neutral) robots and then set adrift on their island without ever having heard human voice I think it would be impossible for them not to have some degree of outside influence upon their sexuality. The story of The Blue Lagoon is set in Victorian times when sex was taboo in polite company. The children have an adult caretaker on the island with them when they are younger, but, conveniently to the plot, he dies before telling them about the birds and the bees.
But if there were truly no influences upon two such people I find it at least feasible that they might have more than one level of lovemaking. Actually, I think that many couples do invent some practices, among themselves, that are short of orgasmic intercourse. It may not have a name, it may have nebulous boundaries, but I think that it is not unheard of, even if the couple are not aware that such practices are known outside of their intimate society of two.
And Now, Let's Return To The Real World
In the real world I doubt that such a thing has ever actually happened. Perhaps two innocent children may have, at some time, been castaways on some tropical island, but their chances of survival would have been low. The chance of just one male and one female child being in this circumstance would be low, why not one child, or three, or two girls, or two boys? The story of The Blue Lagoon relies upon a degree of artistic license for its own survival, but it does serve as food for thought.