Steve,
there is more evidence for than against because you can’t prove a negative and when studies are undertaken like the twin studies they consistently find that when one twin is gay the other is gay over 50% of the time, which is way way way above the 3-5% average, It is suggestive of a genetic factor even though we don’t know what. There are no twin studies showing that the likelihood is 3-5% as it is in the general population. Is it conclusive NO but it is suggestive. There are 0 studies showing 3-5% rate in any of the sibling studies. Can we conclusively say it is therefore genetic, NO. My educated guess is that predisposition is inherited and is realized when the environment (most likely the fetal environment) is conducive. Will that ever be proved, I don’t imagine that it will in my lifetime and perhaps never.
In my own family I had a great uncle who was gay, an aunt who was lesbian, I have living, a lesbian niece, a gay nephew and a young grandnephew who I think may be gay. Is that conclusive evidence NO, It is a single family so I can’t generalize from it but it certainly makes me wonder.
The exact same DNA in recent twin studies has shown that the DNA is not as exact as previous generations of geneticist though. From Scientific America:
Identical Twins’ Genes Are Not Identical Do a search on “how exact is the DNA of identical twin” Will it prove that there is a gay gene, NO but hopefully it will give people here a better understanding of the complexity in genetic research. Nothing is simple in a complex world.