Pretty much all people against gay rights believe that it is a choice. If not, they're just morons.
And people are against them because some humans don't like too much change and allowing homosexuals the same rights as heterosexuals is a very big step for the world, particularly America. Not as big of a challenge as the struggle for rights for African Americans, but loosely the same concept as people are harassed, stereotyped, and sometimes even killed for it. In a few decades, I have a feeling it'll be much more widely accepted, and while homophobic individuals will still be out there of course, there will be a lot fewer of them.