Evolution hardly ever has a direct relationship with population control. Evolution has no purpose, it is simply a process. It has no way to know that homosexuality, in this example, could/should/will help the species in the long run. At least in the gene-centered view of evolution. This theory might hold some weight in group selection, but group selection as a whole is still a debatable topic in which biologists seem to disagree. The majority are not convinced that it is something that occurs.
A larger population is usually beneficial for the species until the point where members of the same species start competing for resources. When this happens, the whole community and then the ecosystem start to destabilize. This fluctuations usually occur naturally for some reason or another, last for a relatively short period and then the ecosystem starts to stabilize again. The constancy of an ecosystem measures how much it can remain unchanged, the resistance how much it can resist to perturbations, including population fluctuations, and the resiliency measures how long it takes an ecosystem to stabilize itself after a perturbation. This are the three main ways to measure ecological stability.
Populations fluctuate for various reasons, being food supply, as Kanzler said, the most important and common one. A particularly bad drought season will reduce the population of every species in the ecosystem, most importantly the carnivores, due to a reduced food supply. However the inverse also holds true, a particularly bountiful season could skyrocket the population of many species. This are obviously the most basic of examples. A particularly humid season could favor the growth of fungi, which in turn could hinder the growth of some plants. This could have many possible ramifications, a reduction in the population of said plants and their consumers and the proliferation of another species of plants and the animals that consume it, for example.
The metapopulation is a group of connected populations in a region. immigration and emigration is common between these populations and it's not uncommon that because of the destabilization, some of these regional populations goes extinct. The population dies or emigrates in that particular region because of the destabilization of the ecosystem and members of the metapopulation immigrate there once the ecosystem stabilizes itself.
If the metapopulation of one of these species is threatened, then the species can become extinct in the region, which in turn would drastically change all of the ecosystems permanently.
This video shows brilliantly how a species disappearing, or rather a species returning after disappearing, changes the ecosystem and stabilizes the animal populations.
Malthus' exponential law of population growth helps predict how populations will behave when no external influence is presented to the ecosystem. From this and other laws, many mathematical models have been created to analyze and predict how populations will react in different circumstances accounting for many different variables.
Even with so many different variables, I don't think homosexuality is prominent enough to make a significant impact on animal populations.