If you mean, familial bonding, divorce rates, crime rates, etc., things that can be measured and linked to marriage, then you have absolutely no claim to harm done by SSM. All of those problems are not only found in Heterosexual Marriages, they are found in higher amounts. If that's "the institution of marriage", then the Gays are the best thing that could happen to it.
If "the institution of marriage", is just whatever the hell you want it to be at the time you make the argument, what's the difference between that and just your opinion? Unless it's an objective, quantifiable, and rational metric for judging the "harm" or absence there of, it's a meaningless argument. If you can't define it, then it's inherently meaningless.