In evolutionary terms, there is strong pressure for a male/female ratio of 1:1. If you have twice as many females as you have males, producing more male offspring is a much better way of ensuring your genes get passed on (your male offsping have a much metter chance of fathering more children themselves than a female child would). If you have more males than females, producing female offspring is beneficial - many of the males will not get the opportunity to breed, while all the females will. Equilibrium is reached at about 1:1, at which point having offspring of either sex is equally advantageous.