There is still uncertainty about the relative importance of genes and environments on human sexual orientation. One reason is that previous studies employed self-selected, opportunistic, or small population-based samples. We used data from a truly population-based 2005-2006 survey of all adult twins (20-47 years) in Sweden to conduct the largest twin study of same-sex sexual behavior attempted so far. We performed biometric modeling with data on any and total number of lifetime same-sex sexual partners, respectively. The analyses were conducted separately by sex. Twin resemblance was moderate for the 3,826 studied monozygotic and dizygotic same-sex twin pairs. Biometric modeling revealed that, in men, genetic effects explained .34-.39 of the variance, the shared environment .00, and the individual-specific environment .61-.66 of the variance. Corresponding estimates among women were .18-.19 for genetic factors, .16-.17 for shared environmental, and 64-.66 for unique environmental factors. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior.
ScienceDaily has an important caveat:
...The individual's unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups, and sexual experiences.
In plain language these results suggest that the largest proportion of the variance within the population for this trait (same sex orientation) can not be accounted for by variation in genes or shared family outlook/experiences. They also shouldn't be surprising, Judith Rich Harris has written two books, The Nurture Assumption and No Two Alike, attempting to explain the "non-shared environmental" aspect of variance which pops up in behavior genetic studies (the breakdown is often on the order of 50% genetic, 10% shared environment, and 40% non-shared environment). Do note that that component of variance may still have biological underpinnings; e.g., an environmental shock during the prenatal stage which changes the path of development. Finally, I do think it is intriguing that females show a much larger contribution of shared environment. To me this dovetails with the anecdotal observation that facultative homosexual behavior is more common among females than males, especially when you exclude extreme situations such as imprisonment or living in Saudi Arabia. Also, if you're interested, Gay Men, Straight Women Have Similar Brains. The study will be published in PNAS when they get around to it....