Let’s try it this way:
Are there arguments against bigamy or polygamy that are not rooted in the hatred of people who have multiple wives or husbands? Can you think of ANY? Anyone? Beuhler?
Marriage as the legal and moral foundation of the family unit is a pretty strong element of our cultural structure. There are arguments against messing with it that are essentially rooted in cultural conservatism, which says that you don’t mess with a culture that works unless there is a very, very compelling reason to do so. There are unintended consequences. Just as there were unintended consequences to other social changes - everything from the pill to women’s emancipation to the liberating aspects of the automobile on teen life to the rise of two income families and the widespread use of day care. People make the argument that expanding the definition of marriage weakens it and waters it down, and makes it less useful as a social construct. And since conservatives tend to see family and community as the core of society, this worries them. They don’t have to be homophobic to hold these beliefs. Many of the people against gay marriage are fully in favor of civil unions - they just don’t want the institution of marriage messed with.
And before anyone jumps down my throat, I have to reiterate that I support gay marriage, and not just by voting. I helped set up a gay wedding for friends, and I was their photographer as well. But I recognize that the people who are against it are not all evil homophobic bastards. I have respect for their opinions.
Except for the homophobic bastard opinions. Let’s not water down the meaning of the word homophobe by applying it willy-nilly to anyone who disagrees with you. That lets the real homophobes off the hook and lets them blend into the crowd.