"Theories on the Origin of Sexual Orientation, 1864-present"

I've found a very good aggregation of the various theories of the origin of homosexual attraction.

1896 -- Published in a German pamphlet, "Sappho and Socrates," which describes the origin of homosexuality as taking place in a bisexual embryo.

Hirschfeld posited the existence, in the embryos of both sexes, of rudimentary neural centers for attraction to both males and females. In most male fetuses, the center for attraction to women developed, while the center for attraction to males regressed, and vice versa for female fetuses. In fetuses destined to become homosexual, on the other hand, the opposite developmental sequence took place.

While admitting that the location of these centers was still unknown, Hirschfeld predicted that when they were identified, it would be found that adults of each sex carried the vestigial remnants of the centers typical for the other sex.

I wouldn't be surprised if that turned out to be true.

1978 -- [E.O. Wilson] hypothesized a possible genetic predisposition for homosexuality in certain humans by using a theory he calls 'inclusive fitness,' defined as the sum of the individual's reproductive successes plus the reproductive success of others who carry that person's genes. He explained that there are homosexual genes that exist not only in the individual who is homosexual but in his relatives. Homosexual persons contributed to the survival of the family by not having children so they were available to support and help other family members, by serving in roles such as aunt, uncle, shaman, or medicine man. Thus, genes for homosexual orientation increased in frequency, not because they aided the homosexual person in his or her own survival but because they aided the relatives who shared his gene pool. This broader spread of the genes helps explain how persons with the homosexual genes could be reproduced, since they themselves often did not produce offspring.

It may also just be an ameliorating side-effect.

2012 -- [...] For several years, studies led by Andrea Camperio Ciani at the University of Padova in Italy and others have found that mothers and maternal aunts of gay men tend to have significantly more offspring than the maternal relatives of straight men...

[...] The theory holds that the same genetic factors that induce gayness in males also promote fecundity (high reproductive success) in those males' female maternal relatives. Through this trade-off, the maternal relatives' 'gay man genes,' though they aren't expressed as such, tend to get passed to future generations in spite of their tendency to make their male inheritors gay...

[...] Turns out, the moms and aunts of gay men have an advantage over the moms and aunts of straight men for several reasons: They are more fertile, displaying fewer gynecological disorders or complications during pregnancy; they are more extroverted, as well as funnier, happier and more relaxed; and they have fewer family problems and social anxieties.

Unfortunately, there is no such breadth of reasonable neurological and biological theories for consanguinamory. Only GSA is understood, and even then I think scientists assume more understanding than they actually have. I suspect there are many unexamined subtleties in "normal" human attraction and the Westermarck Effect.