Did you mean to say identical twins? Such a study would be far more meaningful.
I do not place as high an emphasis on scientific findings as farmfriend seems to do. Believing science has the answer to social questions is part of the problem of the secular humanists.
People are people. Whether being gay were 100% genetics or 100% choice makes not a whit of difference to me over the two questions panther33 posed.
Does the U.S. government have a right to ban gay marriage?
Can America justify making homosexuality illegal?
Marriage is currently defined as one man, one woman. Mixed marriages were banned. That is correct terminology. But gay marriage is a new concept altogether.
Homosexuality, defined as a sexual preference or orientation, is not illegal anywhere in the fifty states, nor is anyone proposing that it be made illegal. So that is a weird question to pose. Sodomy laws criminalized a behavior. I am opposed to sodomy laws, because I value anyone's freedom to do that in private. I do think it is none of anyone else's business. Making such laws unconstitutional was a very bad SCOTUS decision, however, that will bring a lot of harm.
Regarding my second question, I meant, "Can America justify making gay marriage illegal?"
I, too, couldn't care less about what people do in the privacy of their own bedrooms (unless, of course, it's like rape or something like that, but obviously we're talking about a completely different issue). My stance is that I find it atrocious to suggest that the government put its stamp of approval on gay marriages by issuing marriage licenses to gay couples.