One of the basic laws of free market capitalism is that unprofitable areas become deserted. This can be observed in many regions of the world which have switched from regulated or planned markets to more or less free ones: people move into the rich capital cities at rapid speed.
The same law holds for the sexual market. If social norms and conventions are strong, even males with low sexual market value will find mates. If the sexual market is completely free, females will crowd around the high-value males, creating a strong imbalance.