There are various theories, one of which is colloquially referred to as the "gay uncle" hypothesis, which pertains specifically to kin selection, and explains how a given allele that disfavors reproduction may be sustained in a given population.

For thousands of years, humans persisted in tribes as hunters and gatherers. The adults of a given group are principally responsible for providing protection and nutritional sustenance to the other members of the group. Thus, one could posit that the overall net fitness of the group decreases as a function of viable offspring. That is, as the amount of offspring within a certain group increases, the overall fitness of the group decreases. The mobility of the group decreases, and more food is required for sustenance of the offspring.

However, if a particular allele appears in the group that deters individuals from reproducing, then the ratio of adults to children shifts and the net fitness of the group increases - they're more mobile, they can acquire more food, and they're more likely to survive as they don't have to expend energy caring for entities that can't survive without aid. The allele wouldn't have to be fixed within the population, as that would certainly be detrimental to the fitness of the group, but even a 1/15 chance of acquiring the allele would elevate the overall fitness of the group.

With that said, homosexuality likely isn't defined by a single gene. While genetics certainly appears to play a role, it's likely a constellation of factors that ultimately defines one's sexuality. Environmental factors, psychological factors, and even hormonal factors in utero likely have an effect on one's sexuality.

For example, simply because you have a genomic permutation that results in a proclivity for elevated levels of LDL doesn't mean that you will invariably develop heart disease. Certainly you're at an increased risk, but there are numerous other factors at play. Likewise, with sexuality, one may possess a specific genetic framework that may make them more likely to develop homosexual tendencies, but there will certainly be environmental and biological factors that have an effect as well.

Additionally, the "gay lifestyle" is most certainly a choice, as it's a diacritical cultural indicator that concurrently provides psychological satisfaction by allowing one to conform to a minority archetype. Conversely, possessing intrinsic, immutable sexual desires for members of the same sex is not.

This post was edited on 6/6/14 at 11:19 am