1. Homosexuals by not being inclined to reproduce may now contribute excess resources to the group well-being.

2. Homosexuals can pick up slack child-rearing duties, e.g. in cases of orphaned children, and do so without the complications and child favoring that happens very frequently when a child is adopted by someone with his own offspring.

Furhter contributing: Homosexuality is not strictly binary, it is a combination of factors, so that those who never mate will not contribute too strong of a homosexual orientation to future generations, there are those who are bisexual or "a little gay" who can keep passing the genes that will make some portion of the population homosexual. Cultural norms of course will play in here as well sometimes being more permissive, other times not, but I wouldn't be surprised to find that there is subtle genetic idea of balance that is driving this cultural throttle to the homosexual thing.

The big thing to remember tho, is that evolution doesn't always select on individual basis, but frequently on a group basis.

NOTE: i use active words here, as if evolution has a goal, but that is just because I don't want to write like: the selection pressures sometimes result in an overal systemic balance and species or group wide advantage while at the same time resulting in some individuals with a much lower reproductive probability.

I get the bi-sexuality aspect of this, but homosexuality is much more than simply not being selective in who you have sex with. There are significant traits and brain changes in people that identify as homosexual that shouldn't be present if all homosexuality is simply the result of permissiveness in our ancestors.

While the effects of evolution can be observed on a macro scale, they are always the result of selfish genes. In other words, I'm not sure I completely buy the idea of kin selection, especially with humans. That being said it's not something I've deeply versed in.

The cornucopia of debilitating mental illnesses that we tend to suffer from implies that this is almost certainly the case, and there are a lot of things that go into this.

Regarding traits like homosexuality, it's likely that the genetic "cause" ends up so widespread because the possession of it (or of a close precursor) is not necessarily that devastating to reproductive success, and may even be beneficial when it's not expressed as homosexuality. For instance, it's pretty well established at this point that whatever predisposes people to homosexuality genetically is not the whole story, a lot of it has to do with the embryonic environment, upbringing, and possibly sheer chance, so if there are other beneficial things about that mutation (or set of mutations), then as long as they outweigh the probability that homosexuality is expressed and prevents procreation, the gene stays around.

There's a rule of thumb, "one mutation, one death", which implies that no matter how deleterious a mutation is, on average, it will cause one death (or more accurately in this case, one failure to reproduce) when it happens. Roughly speaking, this is because if it's a very mildly deleterious mutation, it will spread quite far through the population before there's enough evolutionary pressure to stamp it out, and if it's a really bad one, it will kill off the first person to have it before it spreads at all. This can be influenced by social factors, as well, i.e. eyesight problems are rectified by our building glasses, but that just reduces evolutionary pressure against bad eyesight, so we're letting it spread its (much lower) fitness penalty throughout the population.

There's an element there that I'm glossing over, which is that in a population, you have to consider the probability of recreating the deleterious mutation, too - for instance, if a trait depends on a combination of a few mutations to be present, then depending on the number and prevalence of those mutated alleles in the population, the stable percentage of people displaying that trait will change.

It may just be the case that for various social reasons, predisposition to homosexuality has not been responsible for enough of a decrease in reproduction to counterbalance the probability of randomly mixing genes in just the right way to recreate it "from scratch." This could be particularly likely if some of the involved genes are useful by themselves in other situations, and homosexuality is hitchhiking on the backs of those beneficial mutations.

It allows excess parents without having excess children. Assume your objective function is to pass along genetic material that is similar to your own. Depending on your resource constraints of your environment and how close your population is to its carrying capacity, it may be game-theoretically optimal to help raise your sibling's child, rather than produce your own. If your child lowers the resources available per child for all your immediate relatives, you would be better off trying to increase the survival rate of an existing closely-related child instead. Thus, a gene that occasionally produces non-breeders (especially if the chance is modulated by relevant environmental factors) will, when properly tuned, result in a larger number of successful children.

The bigger lesson is, look at the genetic distribution of the population, not necessarily the genetics of the individuals. Humans live in groups and interact with each other, so things that don't make sense for the individual can still work. As an analogy: doctors increase everyone's survival rate, so you want lots of doctors right? Well, no, then no one would make food and we would starve. But that doesn't mean doctors are bad, either. It means you want some percentage of the population to be doctors, and that percentage is neither 0% nor 100%.

Upon sighting a bird of prey, a meerkat will stand bolt-upright and send out a warning call to other meerkats. The meerkat that does this has a demonstrably higher risk of being eaten, but in doing so improves the odds of survival for its close family.

It's not about individual reproduction; it's about the survival of genetic material. Furthermore, genes aren't a rigid blueprint; they're highly affected by the environment and by simple chance; it's not as if "oops, this family evolved a gay gene and now all of their kids will be gay and they will all die out".

Parasites. Parasites frequently manipulate the host to change its behavior. Toxoplasma gondii, for example, is associated with a 6X increase in car crashes among human drivers. Parasites also cause variety to evolve in the host immune system and other body systems, resulting in a variety of oddball genetic variations and autoimmune disorders.

There is also an effect called frequency-dependent selection. Some gene variants are useful if you get one copy of the variant, but harmful if you get two. Evolution multiplies the useful gene in the population to the point where the good effects on reproduction balance the bad. Examples of this include sickle cell anemia (one copy protects against malaria, two copies give crippling sickle cell attacks), and the sphingolipid storage disorders (a small influence causes higher IQ, more influence causes lethal degenerative changes of the brain).