Obviously selection would appear to not favour being homosexual, in an evolutionary sense it represents somewhat of a decrease in fitness: Homosexuals fail to reproduce successfully due to the requirement of both male and female gametes and reproductive organs, therefore significantly fewer than the average heterosexual couple. Certainly I don't think it would have evolved as some kind of population control method - group selection theory is generally discarded in favour of a gene-centred theory (see Dawkins for popular science literature)
There is debate about whether someone can be biologically "preprogrammed" to be homosexual, this can occur genetically or epigenetically. Genetic models have used kin selection, overdominance, sexual antagonism in the past and are briefly discussed in the following article on epigenetics. Recent work has looked at possible models by which heritable homosexuality could arise by epigenetic markers and this was covered in a lot of mainstream media (1,2.. just google epigenetics homosexuality). This quote is from their abstract and explains one reason it is difficult to pin down heritable causes of homosexuality:
Pedigree and twin studies indicate that homosexuality has substantial
heritability in both sexes, yet concordance between identical twins is
low and molecular studies have failed to find associated DNA makers.
This paradoxical pattern calls for an explanation.
By their readily testable model they have shown possible (& plausible) conditions under which the "epi-marks" which cause homosexuality could spread through populations.
Genetic studies of twins where one is homosexual have also revealed links between genes and a mating advantage. For example if in a pair of male identical twins one is homosexual, the other has a mating success greater than that of the average male. However, it could be that this link is found because of social reasons (perhaps someone who has grown up with a homosexual sibling may have a different mentality or lifestyle socially which could help them increase reproductive success).
Note: I have no problem with a person's sexual orientation & the rights/ethics of homosexuality is not the topic of this question & answer. This is purely an answer to the obvious evolutionary conundrum.
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