They aren't really the side that is right, they are just the side that has 99% of people. Gay marriage is contentious because it's practically an even split right now.
In 1986, only around 30% of Americans "approved" of interracial relationships -- now about that number have a family member IN an interracial relationship. Things move faster now, and in just a few years, the idea of being against same-sex marriage will seem just as insane about being against interracial marriage.