I have long heard the argument that homosexuality is "unnatural" and hence inherently inferior to heterosexuality. And it is certainly true that, were all or even a great minority of humans homosexual, the ability of the human race to replace its numbers without extreme technological intervention would be at serious risk. The argument then goes on to say that, therefore, homosexuality is evil or at best some sort of psychological illness.
However, this argument has two major flaws.
The first is that, if homosexuality really was so inferior, one would imagine that it would be strongly selected-against by biological evolution. After all, an abnormality which directly makes one MUCH less likely to successfully breed is almost the definition of an evolutionary handicap. While it's true that some homosexuals (and many bisexuals) do manage to sexually reproduce (through occasional heterosexual acts), it's also certainly true that homosexuality and even bisexuality greatly reduce the expected reproduction rate, without greatly increasing the chance of survival of the offspring. So it is neither a good r- nor k-based evolutionary strategy.
And yet homosexuality and bisexuality are extremely common. These are behavioral continua rather than discrete states, so with this understanding, most estimates show something like 1-5 percent of the population is exclusively homosexual, and 6-10 percent is bisexual. This means that up to 1 in 20 of the population is unwilling to have sex with persons of the opposite sex, and up to 1 in 10 is willing to have sex with persons of both sexes, in terms of orientation (obviously, a monogamous bisexual is having sex with persons of only one sex during the duration of that relationship).
For a "defect" to be that common, either there is something profoundly broken in human sexuality, or it is not as much of an evolutionary disadvantage as it appears.
The reason why I believe that the latter option is true -- that the trait of homosexuality is not the tremendous evolutionary disadvantage that it appears at first glance, is based upon the second problem with the homosexuality-as-defect theory.
This is that in numerous other mammalian species, at most times of the year, homosexuality is normal.
These are species that have fixed mating seasons, and in which the two sexes do not remain together save during the mating season. This means that insofar as they are social animals, they spend most of the year traveling in groups of same-sex adults -- in the case of the females, accompanied by their pre-adolescent children. This is especially common among pinnipeds and ungulates -- creatures which tend toward strong sexual dimorphism and mating polygyny.
Now, sexual attraction is a powerful social force, and evolution frequently uses the strategy of "pre-adaptation" -- which is to say, taking something which has been evolved for one purpose and re-using it for another purpose. What sexual attraction is used for in those species, out of the mating season, is to help socially-bond same-sex groups so that these groups will cooperate in meeting various survival challenges, such as avoiding predators or helping injured members of the group.
How does this work? Most mammals have evolved the tendency to find stimulation of primary and secondary sexual zones pleasurable. This creates an emotional reward, so that the mammals will in season exert themselves to seek out sexual partners.
If, out of season, the members of the same-sex survival group stimulate each other sexually, then this pleasure is added to the emotional bonds originally based on maternal and sibling love, strengthening them and increasing the chance that group members will love one another and hence be willing to exert themselves to protect other members of their group. This increases the chance of survival for all members of the survival group, so that this behavior will be strongly selected for.
There is another advantage of doing so in these polygynous groups. In such groups, only a few males -- generally those in their physical prime -- get to mate each mating season. The males first compete with one another, often in ritual combat, to select who gets to mate. The winners of these contests get to mate with numerous females. The losers do not get to mate at all this season -- and hormonal flows in the males reinforce success and punish failure.
Younger males have not completed their growth, so it is unlikely that they will win such contests. They should try -- because they might win, and in any case will learn ritual combat skills even if they are defeated. But of course, they should not try if they are killed or severely injured in the contest.
This means that they have to know when to give up.
Now, if their only chance at erotic stimulation came from successfully mating females, they might try too hard. They would stand a high chance of dying, or being crippled, and they might cripple worthier males. What's more, their rivals may well be siblings or cousins -- they might even be males who would out of the mating season be their survival-group friends.
This is the second advantage of homosexual affection among these creatures. It serves them as a consolation prize -- they are thus not as likely to kill or cripple themselves through over-exertion in mating contests. This makes it likelier that they will succeed in mating in some future season.
How does this apply to humans? We are not, after all, creatures who normally live in sex-segregated groups save for a specific mating season: we instead normally live with members of the opposite sex in year-round mated groups, once we have attained adulthood.
First of all, homo-erotic bonds can be used to strengthen group cohesion in the specific instances where we do split into sex-segregated groups. Originally, we were hunter-gatherers, with the males mostly hunting and the women mostly gathering, which meant that our typical work day involved splitting into sexually-segregated groups. Obviously, there was a survival advantage in mutual affection within these groups, which meant that members of the same sex could not be regarded merely as rivals for mates, they had to be seen as "mates" in the Australian sense of the word.
Actual gay sex was not required for this. All that was important was that we see close friends of the same sex in somewhat eroticized or (more precisely) romanticized emotional terms -- what the Japanese call nakama -- we needed to love them. Assuming that this tendency was genetically-encoded, that it was encoded by at least one and possibly more genes, it is obvious that it would be possible to receive stronger or weaker doses of this emotional instinct. Under this theory, gays get too strong a dose, and bisexuals an unusually but not overpoweringly strong a dose of the same-sex attraction -- and it isn't selected against evolutionarily, because at average levels the result is that one is motivated to bond with one's team mates.
A second possibility is that the ability to generate homosexuality as a human variation is selected for by kin-based altruism. Consider this: each individual in a group is strongly motivated to support her (and to a lesser extent his) children. That individual will help his own mate and children first, and anyone else second.
But what happens when those who care for the children are killed or injured, or some other crisis demands that a reserve of effort be available now to stave off greaer losses later?
It is extremely likely that the person who helps in such a case will be someone who despite being an adult has NO CHILDREN OF THEIR OWN. And such a person, for obvious reasons, is disproportionately likely to be homosexual or bisexual. In this theory, homosexuality creates an economic reserve for the benefit of the whole community.
Thirdly, homosexuality can serve as a consolation prize, as it does with the pinnipeds and ungulates. Adolescent human males, like most adolescent males, are very motivated to seek out sex but are at a severe disadvantage in competing for it. The situation is not quite as bad in humans, since humans are not strongly polygynous -- but we are WEAKLY polygynous, and in some human cultures it is quite normal for successful human males, often in or beyond the prime of their lives, to have numerous (usually younger) wives, depleting the supply of adolescent human females for the adolescent human males to court.
What do the angry young man do about it? One possibility is that they become extremely violent. This seriously destablizes such societies, unless they can bend this violence to socially-useful ends (such as forming the young men into an army to raid or conquer other societies). Another possibility is that at least some engage in homosexual behavior. (This is probably why homosexuality, while deeply-hated by Muslim religious authorities, is extremely-common in the Muslim world, perhaps even more common than in the liberal West).
I believe that all three theories are correct. The capability for homosexual affect strengthens nakama, it helps generate a creative and economic reserve, and it seves as a consolation prize for men and women unable to find mates.
So let us celebrate the existence of homosexuality, which helps bind our cultures together. May we always have it with us!