I think their point is that the "destroying the institution of marriage" argument is specious. If gay marriage truly did cause harm to the concept of marriage as a whole, it would *have* to be causing harm to individual marriages. Which is why the question "what specific harm is this causing to YOUR marriage"?

Think of it as a scientific experiment, where you're testing for the existence of a condition which is invisible except for its effects on the real world. The only way you can prove it is to look for those effects. If those effects don't exist, chances are your hypothesis is wrong.