This isn’t a social or political question. It’s an evolution question.
Tall people tend to have tall children. Smart people tend to have smart children. So, acccording to evolution, gay people would tend to have gay children. It’s just that gay people, by their very nature, don’t have hetrosexual sex, and so fail to have offspring and pass-on that trait.
But they make great aunts, uncles, and babysitters, who are content to look after their sibling’s kids instead of trying to get their own. So they increase the odds of surviving for their genes, indirectly. This pattern is widespread throughout the animal world, with siblings taking care of younger sibs, or infertile aunts/siblings doing the same with social insects. Homosexual humans would fit the same ecological niche.
Kinda like how cripples have crippled kids? And funny people have funny kids? And those with hare lips have kids with hare lips?
The average fertility of homosexuals isn’t zero. I know dozens of gay people who have biological children.
In other words - gay people sometimes pass on their genes the same way as straight ones do. 
In fact, it’s probably been pretty common for much of history - in societies with more homophobia, people would stay closeted, even to the point of getting married, having kids, and so on.
The answer is that neither genetics nor evolution is as simple as people like to think. In this case, we have, as one possibility, kin selection.
(I’m stipulating here that homosexuality has a genetic component, only because the OP’s question seems to presuppose that. I don’t know that that is the case at all, and if it is the case, I see no reason whatsoever to believe that it is the one explanation for all homosexuality. I also want to point out that while there may be a genetic or biological basis for being homosexual, there is nothing biological or genetic about being gay, which is largely a learned cultural role.)
Imagine, a population of primates divided up into small family-based bands. They are competing with resources with other bands. It’s an evolutionary advantage to them to increase their numbers, but not to increase their numbers to such an extent that they exhaust the area’s resources and starve.
So, an evolutionary trait arises, or more specifically a set of evolutionary traits. Those individuals that have the full set of them are disinclined to reproductive sex, but do grow to full maturity and contribute to the life of the band. Their kin, who do not have the full set of those traits, do reproduce, passing on bits of that set of traits to the next generation. The genetic lines that contain these traits will, over time, have an evolutionary advantage over those that do not, because they will have active individuals that don’t contribute to overbreeding.
While I am not claiming that homosexuality is caused by a recessive gene, I would like to point out that if it were it would be perfectly reasonable for homosexuality to continue to exist even if it provided not= evolutionary benefit. There are plenty of genetic diseases (hemophilia and cystic fibrosis as a couple of examples) that have persisted despite providing no evolutionary benefit and in some cases virtually ensuring that the afflicted individual will not reproduce.
IF homosexuality is genetic (which has not been shown) AND all the people having children are 100% straight (which is not the case), then we would have to look at environmental factors. My guess is that, for whatever reason, a certain percentage of people are born gay and it probably has little to do with the orientation of their parents.
Note: I am not suggesting that, if homosexuality does not have a genetic component, that it is a choice. There are other possibilities including womb environment and epigenetics that could result in straight people having a gay child.
That would only be true if being gay is mostly genetically determined and the genetic link is fairly simple. If, for example, only 50% of the gay population is gay because of genetics, then your argument falls apart. On the other hand, if it’s 100% genetically determined but you need 10 specific gene variants to turn out gay then there’s hardly any selective pressure to reduce any one of those genes - especially of most of those genes turn out to be beneficial in other ways. I don’t think the results are in on this question.
Also, gay people can and do have children. Which again is only relevant when there are specific genetic factor.
Presuming homosexuality is genetic, it wouldn’t be much of a stretch to presume it to be a recessive gene. Once you get to that point, it become like any other recessive gene, which can be passed down through infinite generations even if those who express the gene don’t ever breed.
One theory (neither well-adopted nor definitely rejected, last time I checked) was that if you are Joe Neolithia and you and your wife Susie Neolithia have several kids, and your brother Mike Neolithia is gay and has no wife or kids (and within the confines of this theory, I guess, also has not set up connubial cavekeeping with his permanent partner Tom either), Mike may be staying in your cave and contributing resources including babysitting and family defense and freeing up you and Sue to do productive things (and reproductive things) all of which gives the children of Joe and Sue a small edge in the survival sweepstakes.
It is similarly argued that if Aunt Imogene survives beyond her childbearing years and hangs out around the cave and similary contributes, her longevity (way past the onset of menopause) also gives the Neolithia kids an edge.
ETA: well that will teach me not to refresh the post before replying. When I loaded this page there was only the first post, really!
More to the point, it’s only relatively recently - in the West at least - that people marry for love. Arranged marriages were the norm. Further, gay or not, you needed children to provide for you in your dotage.
my bit of armchair psychobio here is that homosexuality deals with the arousal/attraction portions of the brain. certain biological predispositions can obviously affect which way the attraction swings, but also in development there are factors that mess with how arousal mechanisms are triggered.
thus, there isn’t a necessarily a “gay” gene, but probably a gene that makes people more susceptible to altered attraction/arousal mechanisms. so, people can certainly be a carrier of that gene but by sheer chance their arousals remain strictly hetero.
I’ve made this argument before on these boards. It seems if this is the case then kin selection could not be a reason for homosexuality but, instead, would be an unfortunate result of inheriting that exact combination of genes. I would think that, either way, it would explain the persistence of the trait.
Er, I’d also say it’s only in the last snap of the fingers – on evolutionary timescales – that people have a dotage in which to be provided for. And I don’t believe that cultural institutions such as marriage, arranged or not, have had time to make any meaningful evolutionary impact on the species.
There is a hypothesis that some of the genetic factors that are related to homosexuality do not act the same in both sexes. In one sex, they lead to homosexuality, but in the other, they lead to increased fertility/attractiveness/whatever. So, it’s quite possible that homosexual genes increase genetic fitness on average (or, at least, don’t decrease it).
I believe there is at least some evidence supporting this, that all other things being equal, women with gay brothers have more children than women without.
Joey: “If homo sapiens were, in fact, homo sapiens, is that why they’re extinct?”
Ross: “Joey…Homo sapiens are people.”
Joey: “Hey, I’m not judgin’!”
At our “church”, as it were, i.e. Unitarian Universalists, there are six lesbian couples, all have children, all conceived using the heterosexual method.
Is there some other method?
Also, not everything that is genetic has to do with utility for survival. Some things are just random.
And, suppose there was a gay gene, it would bring back discussions of curing their disease.
While, on the other hand, if there is a religious gene, we can cure that disease as well.
There. No favouritism here, I’ve insulted both sides.