Natural selection doesn't deal with “population control”. It's about survival of the fittest. Those who have the best genetic traits to survive and succeed are the ones who get to pass those traits on to future generations, insuring that the future generations will tend to predominantly possess the best traits that allowed their ancestors to survive and succeed. From this point of view, homosexuality is not a trait of the fittest. It's a trait that very specifically tends to prevent those who have it from passing their genetics on to future generations. Therefore, it is a trait that is, and always will be, uncommon, found in only a small minority of any given population.
And in too small a minority to have any significant effect toward “population control”.