50% of Your Genes Come From Your Mother, 50% from Your Father.
In sexually reproducing species, 50% of all the genes passed to the next generation come from a single sex.
If a sex is underrepresented, individuals of that sex pass on a disproportionate amount of their genes.
For most populations, therefore, unbalanced sex ratios do not last long.
Any individual that produces a large number of the uncommon sex or switches to the uncommon sex has a selective advantage.