fgoodwin, I think you may have an incomplete understanding of what genetically based means. As DanKroh said, its not only what genes are there, but what factors influence them. (DanKroh, I dont think its just in-utero factors, either.) Not all genes become expressed.
In other words, even if the hypothesis all homosexuality is genetically caused is true, it does not follow that every homosexuality gene causes homosexuality. Therefore, a less-than-100% correspondence in twin studies does not negate the hypothesis.
I tend to think of it this way: A gene is like a ball on a mountaintop. It might sit there forever or it might get nudged and roll down the hill. If it rolls down the hill (i.e., becomes expressed), the mountain (i.e., phenotype) will look different. Things that can trigger the gene are hormones (in-utero and ex-utero) and, I maintain, viruses.
For example, I think a gene codes for juvenile diabetes. However, merely having the gene is not enough to actually get diabetes. Something has to come along to trigger it. I wager its a virus that does the triggering. (Id like to know whether twins have been studied in this context and, if so, what resulted.)
I wager the same is true for homosexuality. We already know there is a continuum of gender psysiologymost men possess male genetalia and reproductive anatomy and most women possess female, but some possess both or a blend. (You can look it up.) It is not so hard, then, to infer that sexuality follows a similar continuum.
I agree with Trevorumin ten years well know a lot more than we do now about this issue. If I turn out to be right about viruses triggering the diabetes gene youll all owe me a dollar.
Adam
Cubmaster and retired clinical chemist