There was an episode of the original Star Trek in which a primitive society is ruthlessly kept primitive by a nefarious computer (whose outward form was a big lizard-head thing). Then Kirk comes along and says, "The hell with this!", and destroys the nefarious computer. (When someone objects that he's breaking the Prime Directive, Kirk replies, "That refers to interfering with developing cultures. This one's not developing, it's stagnating.")
Then, just before the episode ends, it becomes clear that these poor people have absolutely no idea what sex is, and Kirk just kind of smirks and says, "You'll figure that out on your own." But apparently we can't assume that they will, after all:
"A German couple who went to a fertility clinic after eight years of marriage have found out why they are still childless - they weren't having sex."
Via PZ Myers, who wonders what their reaction will be like when they finally realize what they have to do. I personally am really curious about what kind of religious education manages to produce adults who are completely ignorant of "how the plumbing works".