Why does homosexuality persist in the population? If gay people are unlikely to have their own children, shouldn't this trait die out? Alternatively, as gay-rights opponents say "if everyone were gay the species would die out!" so if we accept gays, does that mean the demise of humanity?
Again, let us look at left-handedness. For many years, it was considered flatly wrong to be left handed. Indeed, the Latin root of "sinister" means "left" and people pointed to Biblical strictures against using the left hand. In some cultures, to use the left hand in public is an egregious insult. In my parents' generation, lefties were actually forced to use their right hands, often leading to cognitive and emotional problems; stuttering was a common side effect of this. We became more enlightened as a society and stopped forcing right-handedness on natural lefties; they do have some minor disadvantages (scissors, for example, are typically designed for the right hand) but we all co-exist perfectly happily together.
Okay, then why hasn't it died out? There is no strong selective disadvantage either; moreover, lots of right-handed people carry the gene that "allows" lefthandedness (R/r). And lefties are still able to have children at normal rates which keeps the trait present
A-HA, says the anti-gay-rights viewer, but there IS a disadvantage to being gay, they can't breed! Well, first of all, many gay people do have their own children, either from a previous straight marriage, or with a surrogate. But even for childless gays, as long as any "gay genes" persist in relatives who have children without any disadvantage (think right handed people with Rr genotypes) then there is always a reservoir. And, if there is some advantage to the heterozygote, there can be positive selection in its favor. For example, some studies suggest that straight siblings of gay folks may have higher fertility than people without a gay relative. Simply put, there may be a significant advantage to having an uncle who doesn't have his own kids, because he contributes to the welfare of yours.
In fact, even a negative trait can have a positive selection. A great example is in the gene for sickle cell anemia, which predominantly affects people of African descent. This is a recessive gene such that affected individuals are aa in genotype; thus Mom and Dad were each Aa carriers. Interestingly, you would expect that this gene would have died out in a pre-medical age given the potential lethality of this disease, but it hasn't. Indeed, there is evidence that Aa carriers actually have an advantage because they are more resistant to malaria, and since 75% of their children, on average, will not be affected by the disease (they will be AA or Aa), this is sufficient positive selection to keep the gene present in the population. The a allele neither took over the population, nor died out.
So, an important point to remember is that "Sexual Selection" actually works at the level of a population, not at the level of an individual. For animals with complex societies, it is likely that there are multiple components working towards fitness, and sexual behavior is not limited to procreation but also plays a role in the relationships that are essential to survival (A great discussion of this is in Joan Roughgarden's excellent book, Evolution's Rainbow).
Just to push this further, if you have a complex family structure, it may be to the advantage of the group to have a few offspring to whose welfare many adults contribute--the gay uncle, in our own terms. But consider in a wolfpack, only one pair breeds while the others contribute to care for the pups. Or some species of birds, where a third adult helps the breeding pair raise the chicks.
So, traits can be selected in the population even when they don't improve the reproductive fitness of the individual.
To read this entire series in order, visit the Genetics Page.