[DSHambone]

From the study interpretation you cited:
These results give reason to believe that there is some constitutional component to male homosexuality. However, the twin data are consistent not only with a purely genetic explanation, but also with one involving possible differences in the degree of shared prenatal environment between monozygotic and dizygotic twins (as explained earlier, monozygotic twins experience higher similarity in foetal hormone production, both in timing and in amount, than do dizygotic twins). Some recent theories of the genesis of homosexuality, to be mentioned in the next section, place critical importance on hormone levels in the prenatal environment of an individual. If such theories are true, then the difference in concordance rates between monozygotic and dizygotic twins could be explained largely in these terms (see next section). It should be noted that such an explanation still relies on genetically controlled prenatal hormone production to account for observed differences in concordance between monozygotic and dizygotic twins.

http://www.dai.ed.ac.uk/homes/timt/papers/twin_studies/analysis.html

From an interesting article:

The rejection of a genetic cause of homosexuality by essentially all conservative Christians and some others seem to be based upon a faulty knowledge of the inner workings of genetics. Genes have a property called penetrance, which is a measure of their effectiveness, or power. Consider the gene for Huntington's Disease. It has two alleles (varieties). One is a very rare allele will causes the person to fall victim to the disease. The other, much more common, variety prevents the disease. This particular gene is 100% penetrant. If you were born with the allele that causes the disease, then you are certain to develop the disease later in life. The penetrance of the gene which causes Type 1 (early onset) diabetes is only 30%. So, if one identical twin has the allele that causes diabetes, then the other twin will have the same allele. Both will have a 30% chance of developing the disorder. Both twins will have the identical allele, but it may or may not be triggered by something in the environment, and cause diabetes. Similarly, if one identical twin develops schizophrenia, the other twin has over a 60% chance of also developing the disorder. If one twin develops bipolar affective disorder, the other twin's chances are about 60%.

One theory that fits the available observations is that the penetrance of the "gay gene(s)" is approximately 67%. That causes half of the males with the gene(s) become gay. "There could be hundreds of millions of straight men walking around with this gay allele but who are straight simply because it didn't penetrate" In the case of the "gay gene(s)" perhaps 10% or more of all males have the allele that causes homosexuality, but in many cases is the allele not "triggered".

Assuming that the penetrance of the "gay gene" or "gay genes" is 67%, then one would expect that if one fraternal twin was gay that the other would also be gay about 22% of the time. This number also agrees with studies of families with twins.

Nobody knows what triggers the allele. It might be some event happening in the womb, like an abnormal amount or irregular timing of hormones. It might be some event during early childhood. Either way, it is outside the control of the individual and his family of origin.

http://www.religioustolerance.org/hom_caus3.htm

[capitan_refugio]

It has been over a decade (Hamer, et al., 1993, Science) since a serious article purported to identify a "gay gene." Since that time, there have been several other publications which called Hamer's work into question or have concluded that there is no good evidence for such a gene. Today, behavioral, rather than genetic, reasons for homosexuality are most commonly cited.

I read several general science journals on a regular basis, but I am not geneticist. So I can only pass along what I've read, with no real insight of my own.

All studies in human behavior must take into account cultural and societal mores. If one looks at a close human relative (the bonobo, or pygmy chimpanzee), one might conclude that it is well within the scope of "normal" primate behavior to have sex with just about anything (male-female, female-female, male-male, autoeroticism, inanimate objects, other species, etc.). However, it appears from our more primative "cousins" that complete aversion to the opposite sex is not normal behavior.

Separated twins studies give us some insight into the role of genetics in controlling human behavior. However, the data does not, to date, support the existance of a "gay gene."