I prefer the term 'malfunctioning' myself, but yes, I understand your meaning. I once got into a very amusing argument with a friend when talking about gays in the future. I mentioned quite off handedly that there won't be any gays in the future, because whatever malfunction is the actual root cause will be treatable, either before birth or afterwards, and that no one would elect to be gay if they had the option. She literally looked at me like I just had derailed a passenger train off of a bridge.
All that aside, it's not yet the case today. The long and short of our current situation is that, nature or nurture, adult homosexuals aren't physically interested in members of the opposite sex.
As a political issue, and regardless of the origin, most Americans don't think that homosexuality is something that can be overcome at this time. The context they're going to approach the issue with is 'these people are fundamentally gay, and we have to ensure they are treated the same as the rest of us'.
Knowing that, it seems that there are a lots of good and valid arguments against gay marriage, but the 'homosexuality is a choice' angle is simply not operative in modern American society. Most people don't believe it's a choice, and even people that believe my theory that it's a neurological malfunction don't see the relevance.
All that aside, it's not yet the case today. The long and short of our current situation is that, nature or nurture, adult homosexuals aren't physically interested in members of the opposite sex.
Many people who identify (at any given moment) as homosexual have had male/female relations in the past, or intermittently, and many change and give up the "gay" life. That is why finding fixed numbers of how many people are homosexual is impossible. The numbers change all the time. Many adult homosexuals have been interested in members of the opposite sex in the past, or every now and then.
Your assertion that "most Americans don't think homosexuality is something that can be overcome" is your opinion only. And if "most Americans" read the truth, which is hidden by the media, schools, universities, and government - that homosexuals can and do change - then even more Americans would know that change is indeed possible. So a lot of people (who knows how many?) believe lying propaganda is no surprise, but it is regrettable. That is why getting the truth out there is so important.