Here's my issue. The United States was founded on the principals that people can do whatever they want: illegality starts when someone's actions can somehow harm another person (though many laws, such as helmet and seat belt laws, now violate this basic principle). Following this logic, it would seem at the surface level that gay marriage should be allowed. I personally have something against homosexuality on moral grounds, but please spare the anti-religion, "you're-so-intolerant" crap: I don't have anything against gays or lesbians, I simply disagree with their lifestyle. But I do know that this country was formed so people would not have to conform to a particular worldview, so if they want to live that life, I'm fine with that.
Issue is, there are still plenty of people who don't agree with it. I know a lot of people, myself included, who would not want homosexuality to be taught as normal to my children all their life: many, many cases have already proven that it is next to impossible to raise someone one way when all of society is strongly (and in many cases, hatefully) pushing in the other direction.
So personally, my problem with the legalization of gay marriage has nothing to do with whether or not gay people should be together: rather, it has to do with the unquestioning (which eventually leads to forceful, as we've seen in societies past and even that of the current United States) acceptance of it by society as a whole.
But I will not deny that it is inevitable: out society is drifting away from its roots, which is good in some cases, but bad in most. So we'll soon enough see how a society with absolute freedom works out. My comment on that is simple: Rapture (referring to the city in Bioshock).