As far as the nature thing goes, it'd be simpler to just remove sexual tendancies if there's an overpopulation. However, nature tends to simplify this further: if a place gets too overpopulated animals just stop having offspring all together, not become gay.
Except that humans are an entirely different breed, cognatively. I don't think anyone can claim to know exactly how evolution would evidence itself in humanity, particularly on a global scale.
Overall, my point is.... I think that homosexuality is intended. For whatever reason. Perhaps, once upon a time, something like homosexuality would have to have been considered taboo to ensure the growth of the human race.
In this day and age, I do not see ANY threat to the human race by homosexuality, and so I fail to see how it can be considered a moral issue any longer. To be moral, it has to have some sort of negative implications (not just "I think it's gross", since I think muscular men are gross, but it doesn't make them morally reprehensible). I just don't see it.