In ancient times people weren't just male or female, but one of three types: male/male, male/female, or female/female. In other words, each person was made out of the components of two people. Everyone was happy with this arrangement and never really gave it much thought. But then God took a knife and cut everybody in half, right down the middle. So after that the world was divided into male and female, the upshot being that people spend their time running around trying to locate their missing other half.