"To me, from an evolutionary(and genetic) standpoint, it would make sense that gays would become more naturally prevalent as the world becomes more and more overpopulated."
To me, as an evolutionary biologist, that viewpoint is entirely misguided. Evolution is not forward looking, and it does not favor variants of genes which do not propagate themselves.
Imagine, for the sake of argument, that there's a version of a gene that causes individuals not to breed when the population density is really high. If a mutation arises in a single individual in this population, causing him to have children even though the population density is high, that individual will end up having more children than average in the population. Consequently, any genetic variants he has will be passed along more often than genetic variants others have--meaning that this new tendency to have kids even though the population density is high will spread. Conversely, if there's no tendency to avoid having kids just because the population density is too high, a new mutation that causes someone to limit his reproduction won't be favored, as it will just mean that one individual doesn't have children and thus the genetic variant is lost. Even if the population as a whole would be better off if individuals didn't have too many kids, each individual gene will benefit if it can cause the individual carrying it to reproduce in such a way that it gets the most copies of itself possible into future generation.
As for the root causes of homosexuality in humans, the evidence on region xq28 is shaky at best-- in some studies it looks significant, in others it doesn't rise above the levels of random chance. The explanation with the greatest replicability across studies is birth order for males: the more previous male pregnancies the birth mother had, regardless of whether the child is raised by her or adopted by someone else, the greater the probability that the boy will grow up to be gay. It is hypothesized that the mother's body reacts immunologically to the male fetus, particularly to the higher testosterone levels it produces, and that this response can have an impact on development. That doesn't appear to be the case with lesbians, however. It is important to note, however, that this is a great example of how something can be biological without being genetic.