That's because the "institution" of marriage is not a specific, tangible thing. It's a concept, and it's a concept that means different things to different people. You can't cause measurable harm to a concept. Someone else's divorce has no effect on my marriage. Now apply this line of thinking to same-sex marriage.
This is one of the many, many fundamental flaws in the arguments made by the anti-equality side. They simply can't wrap their heads around the idea that same-sex marriage doesn't harm them or anyone else. They personally disagree with it, therefore it must harm something, right?
If you can't identify any specific harm caused to anything, then why on earth should I believe there's a problem?