It remains unclear whether sexual orientation is genetically determined. Even if it is, that doesn’t justify advocacy for same-sex marriage.
A growing laissez-faire libertarian attitude toward social issues among Americans is arguably the most important weapon available to same-sex marriage advocates. Certainly there are LGBT militants with unwavering commitment to the issue, but the vast majority of those who “support” same-sex marriage can hardly be said to support anything. They just have a hard time saying no. They much prefer the sanitary hands off approach—let them live out their sexuality as long as I don’t have to get involved.