Think the idea that a stork brings babies to parents is strange? Here's how pregnancy worked on the Trobriand Islands: First, someone died. Then he became a baloma (what we might call a ghost) and chilled on the Island of the Dead.
Eventually he got bored of the whole immortal island paradise thing, so he decided to transmogrify himself into an infant and zap himself into the womb of one of his female descendants.
How exactly the ghost-baby got from point A to point B isn't clear, but it's just one of many bizarre cultural beliefs about sex that have popped up around the world.