Every time the subject of "the institution of marriage" comes up, I get the feeling that opponents of gay marriage think that "the institution of marriage" has remained unchanged since time immemorial.
At various times in our history, not so long ago* marriage, for instance, meant the man had legal guardianship over his wife who was supposed to obey him in everything, "until death do us part" was so strict that not even rape or physical abuse was considered ground for divorce, arranged or convenient marriages were common, and interracial or inter-faith marriage was not only considered unnatural but also illegal.
* Hell, in many parts of the world it's still like that.
If "the institution of marriage" survived the idea of marrying whoever you wanted regardless of class, race, or the wishes of your parents, simply because you loved them and wanted to spend your life with them as equal partners, then I don't see why extending the same right to gays will suddenly tear it down.
And if it hasn't survived that, as I know some people have argued, then maybe it shouldn't.