Some have suggested that homosexuality has evolved as a form of natural population control. There are several problems with this idea.

Population control is not necessarily a good thing for the survival and propagation of a population. If there is a population of humans, such as a tribe or a country, Overpopulation will create demand for resources. Because of this, a populous group of organisms will be driven to conquer other areas, and would then have more resources, which would then further increase said group's population. Ergo, groups that can more rapidly increase their population are more evolutionarily successful.

An exception to this are isolated groups, such as those on islands, that cannot conquer other groups of people, are not more evolutionarily successful by overpopulating. Overpopulation within an isolated population can be disastrous, such as with what happened on Easter Island.

Another problem with this theory is that genes for homosexuality within a population do not necessarily cause a lower rate of reproduction. An example of this is the fa'afafine of Samoa. The fa'afafine are a transgender group in Samoa who are biologically male, and almost exclusively homosexual. The fa'afafine practice a form of kin selection as a type of "super uncles", putting a large amount of effort into caring for the children of their relatives. Ergo, the propagation of the genes that cause this breaks even, and these genes that cause homosexuality can still exist.[1]

But wait

Gay soldiers who love each other sexually are more willing to take risks for each other and cooperate better in battle. Therefore a population that includes some gays can win war more easily.
And it took so long before the USA allowed gays into the military.

See also