The girls aren't real. They are being manufactured somehow. Either by magic or science, or a science so advance it is like magic. I assume they are created at a steady rate, and because they all die at 20, they also die at a steady rate. Also, because they are being manufactured, if we consider what we men are trying to do right now (biologically engineer, or even mechanically engineer, the "perfect woman") it might some more sense. Meaning, that they may be programmed or specifically created to crave sex with men.