"The concern of opponents is that they don't want the state actively certifying and promoting something they feel is immoral."
What I'm saying is that these days you can't argue that something should not be legal just because you feel that is immoral, you have to provide evidence that it would bring harm to society if it were legalized. Most young people today think no harm would be done if gay marriage was legalized. So that's why young people are usually against gay marriage.
The reason I brought up incest and polygamy is because these also would not harm society, so we really have, under the "laws should prevent harm" doctrine, no reason for these things to be illegal, which is why I predict that one day, if enough people agitate to be allowed to have those kinds of marriages, they will be allowed to.
Ultimately the "definition of marriage" is something that has changed drastically over time and changes hugely from culture to culture. Nations tend to have laws that reflect their culture, so as long as the culture of the United States is evolving into in which gay couples are accepted as normal, which it is, gay marriage will become legal.
The bottom line is that you have a lot of gay couples who, thanks to decades of work by the gay community, now feel confident enough to demand that the state recognize their marriages, and a lot of people, because of the changed culture, see no good reason to prevent them. If you're not a religious fundamentalist of one stripe or another, or just a random non-religious homophobe, you really don't have any reason to argue gay marriage should not be legalized, especially if you know gay people who are perfectly stable and normal.