Yes, heterosexuals have done a fine job destroying the institution of marriage without any help from gay people. I ask anti-gay marriage people, "Well, I've been married and divorced a number of times, how has it cheapened YOUR relationship with your spouse?" I get some mumble-mouthed answer, then come back with, "Your marriage is your responsibility, you and your spouse are the ones who determine its strength or weakness, others outside your marriage will have no influence on it, unless one of you lets them."
But seriously, other than the far right wing, there is a sense of palpable unease about gay marriage in the minds of the mushy middle. They were fine with civil unions, but at this point, gay marriage evokes an "Integration is fine, but not in my back yard," kind of view. The mushy middle doesn't have too much problem with gay couples who want to commit to each other, but to have them call it marriage is a bit too threatening, right now.
In a way, the Massachusetts SJC decision might have dealt the gay marriage movement a setback. If civil unions had been substituted, and other "domestic partnership" legislation of the kind we've seen in CA, NJ, and elsewhere, there might have been a natural move towards gay marriage in about five or ten years. The suddenness of the MA ruling shocked a lot of people, and the various marriage amendment votes coming up this fall are going to take quick advantage of that shock.
Please, don't get me wrong, I'm not defending bigotry among the mushy middle, but I think I understand it. The right understands it, and they have moved quickly to exploit it.