Example: a population is 70% female and 30% male. Also, there is genetic variation for the number of male offspring a female tends to produce: some females have clutches with more males relative to females and they pass this tendency to their offspring.
In this scenario, females that produce more male offspring would have a distinct advantage, and pass on more of their genes. For instance, a mutant female producing 30% female and 70% male offspring would have a reproductive advantage until the population became more than 50% males.
In a male biased population, the mutant above has a disadvantage.
Result: 50/50 sex rations predominate in nature