" If homosexuality was genetic, then if one monozygotic, i.e. "identical twin," is homosexual, then virtually 100% of the other twins would also be homosexual. This 1991 study shows that only 52% of the second twins were homosexual. The Australian study was even lower, 38%.
This settles the question now, and forever, homosexuality is NOT genetic.
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
http://www.worldpolicy.org/globalrights/sexorient/twins.html
Better research, however, was based on twins who were recruited for other reasons, and only subsequently asked about their sexual orientation. These are known as "registry" studies, and they similarly gave a concordance rate between identical twins of less than 50%. There have been two major published registry studies (4,5), one based on the Minnesota Registry, the other on the Australian Registry. The larger of the two registry studies is the Australian one, done by Bailey, Martin and others at the University of Queensland. Using the 14,000+ Australian twin collection, they found that if one twin was homosexual, 38% of the time his identical brother was too. For lesbianism the concordance was 30%. Whether 30% or 50% concordance (snowball samples), all the studies agree it is clearly not 100%.
The critical factor is that if one identical twin is homosexual, only sometimes is the co-twin homosexual.
There is no argument about this in the scientific community.
http://www.narth.com/docs/whitehead2.html
The distribution of sexual orientation among cotwins of the monozygotic probands appeared to be bimodal. In other words, most subjects classified themselves as heterosexual or homosexual, with very few giving evidence of significant bisexuality. This finding is in agreement with other recent studies (e.g. Buhrich
et al. 1991).
According to their data, 52% (29/56) of monozygotic cotwins, 22% (12/54) of dizygotic cotwins, and 11% (6/57) of adoptive brothers were homosexual. Heritabilities of homosexuality were calculated using these results under a wide range of assumptions of the population base rate and ascertainment bias. Under all conditions considered, heritabilities were substantial (h2 was between .31 and .74 in all cases). However, "the rate of homosexuality among nontwin biological siblings, as reported by probands, 9.2% (13/142), was significantly lower than would be predicted by a simple genetic hypothesis and other published reports."
http://www.tim-taylor.com/papers/twin_studies/studies.html#sotmtsoh