Basically the study found that if one Twin was Homosexual, the chance that other was a Homosexual was the same as any other random person (i.e. the second twin was as likely to be Homosexual as if you would pick anyone else in the general population and find that person was a Homosexual).http://www.worldpolicy.org/globalrights/sexorient/twins.htmlTwin studies and homosexuality
Bailey and Pillard (1991): occurrence of homosexuality among brothers
52% of identical (monozygotic) twins of homosexual men were likewise homosexual
22% of fraternal (dizygotic) twins were likewise homosexual
11% of adoptive brothers of homosexual men were likewise homosexual
J.M. Bailey and R.C. Pillard, “A genetic study of male sexual orientation,” Archives of General Psychiatry, vol. 48:1089-1096, December 1991.
Bailey and Pillard (1993): occurrence of homosexuality among sisters
48% of identical (monozygotic) twins of homosexual women were likewise homosexual (lesbian)
16% of fraternal (dizygotic) twins were likewise homosexual
6% of adoptive sisters of homosexual women were likewise homosexual
Bailey, J. M. and D. S. Benishay (1993), “Familial Aggregation of Female Sexual Orientation,” American Journal of Psychiatry 150(2): 272-277.
The only study I have read on Homosexuality and inheritance was a find that there was a slight increase in homosexuals among second born sons.http://www.findarticles.com/p/articles/mi_m2372/is_4_37/ai_72272310/pg_4http://www.findarticles.com/p/articles/mi_m2372/is_4_37/ai_72272310/pg_1++++
There is so much else in your post to respond to, HappySlug, it's hard to know where to begin.
I have to admit I'm not a believer in any of the psychosocial origination theories, let alone the idea that homosexuality was the norm among primate populations or human societies (with a few exceptions). The correlation of homosexual people coming from families/parents with abusive sexual behavior and unsustainable notions of what is 'normal' fits reasonably well with a congenital/genetic theory of origination.
In some well-studied animals there is some knowledge of how small gender-specific neuron networks and regions of the brain are given the information which of the two forms of development they are going to follow. It's done by the dosage of a protein secreted by surrounding tissue and is dependent on the number of X chromosomes the average cell has. It happens remarkably early in development- maybe the equivalent of the second week of development in humans. Male is specified by "low" amount, Female by "high" levels.
In humans the equivalent protein messenger molecule isn't clearly identified. But it looks a lot like immune system protein messenger molecules known in humans, and it shows up in the immune system functions and limb development of these animals.
After that it's easy to see that if things work the same way in human development (about a 99% chance), male homosexuality could be most easily be caused by male foeti getting exposure to an unusually high dose of this type of protein in its brain during a small window in time of development. The easiest way it could happen is that there is a bacterial infection or other immune system activating event very close to the foetus- maybe an autoimmune response to male-only antigens- and that would, almost by itself, explain the birth order phenomenon and the genetic data linking homosexuality to the X chromosome. (The genetics would be explained by the mother being more susceptible to having this immune response take place than other women- e.g. a genetic alteration that makes her more susceptible to common uterine infections.)
Female homosexuality would occur when there is a diminishing of the amount or effectiveness of that protein factor at that stage of development. This is hard to induce from the outside, so it would have to be due to mutations in genes only slightly affecting their function, but just enough that it is impossible for the "high" level of the protein factor either to occur or to be fully recognized. In other organisms this protein factor has a role in control of limb growth/development. A few years ago, a very careful effort discovered that lesbians have hands that differ slightly in size from those of hetero women, most notably in the fourth finger (aka ring finger) being slightly smaller than expected/seen in hetero women.
Anyway, there are a lot more pieces of evidence that fit well into a model of the kind, which basically says it's all biological and quasi-accidental. But you can predict some things from it- that societies who live in humid climates (West Africans and Greeks, perhaps) have higher rates of homosexuality, for one thing (the uterine infection rate would be higher), and that migrants (e.g. the white colonizers of various contintents) entering climates with large amounts of (to them) novel pathogenic or infective bacteria will also have elevated rates. (Maybe the hot-and-humid American Southeast can be expected to have a rate of male homosexuality significantly higher than the American Northeast or American Southwest, interestingly enough.)
Evolutionarily the prevalence of homosexuality could be explained as a trade-off: rather than destroy foeti when there is a concurrent bacterial infection, many/most are saved by the vigorous but complicated immune response mounted. But the price is that all the complicated immune system signalling leads a proportion of them get gender specified in a way not identical to what their chromosomes would otherwise cause them to be. Over time a balance is found in stable populations that optimizes the ratio of foetal loss to live births (gay and hetero) and (hetero) fertility.