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Mammalian females go through prolonged periods, including gestation and lactation, during which they cannot produce offspring. Males, in contrast, can mate with practically arbitrarily high frequency. Thus, a species that had more females than males would be able to produce more offspring per unit time than a species with the same total numbers but an even distribution between males and females.

Generally, evolution favors not what is good for the species as a whole, but rather genotypes that do better than their competitors. In fact, a mother that produces an excess of the rarer sex (whichever this may be) will be favoured by natural selection. So, the sex ratio will stabilize at an equilibrium where males and females are balanced with equal numbers.

Assuming the males left every available female pregnant, a mutant which created more females than males among its offspring would generate more total offspring in the population. Which genotypes do these "extra" offspring carry? Therein lies the key to the puzzle: the "extra" offspring carry one haplotype (half a genotype) from the mutant that generates more females, and one haplotype from a genotype that generated a male. Thus, for every increase in the population of the mutation coding for more females, there will be an equal and corresponding increase in the population of genes coding for males. And if the ratio did shift toward females, there would be increased selection for genotypes favoring males, since every male would get multiple offspring per gestational period, while each females would get only one. Likewise, if the ratio shifted toward males, each male would on average get less than one offspring per gestational period, while females would get one, favoring females and bringing the ratio back to a 1:1 equilibrium.

So while it would seem at first sight that, if each male could mate with a large number of females, a genotype could produce far more offspring if it produced more males than females, and that the gender ratio should over time have evolved to favor males, if males did start becoming more common, then the average male would not be able to mate with a large number of females. In equilibrium, at a 1:1 ratio, the average male fathers just as many children as the average female.

Here's more information: http://alexbacker.pbworks.com/w/page/1721249/Why-the-Sex-Ratio-is-1

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