The sex ratio of the world is currently 300 females for every 1000 males. As many females were killed when they were just infants. Now, even women mostly give birth to males, as their bodies evolved to not let the most females be a waste if they'll just get killed at infantry. So now, polygamy is illegal and Polyandry is not only legal, but also a must, so there will be more kids. When the females are just in Middle school school, certain males are chosen as per the every female