Indeed. Has anyone posed, seen or heard a coherent answer to this question? How, precisely, does allowing homosexual couples access to the same sorts of legal protections and entanglements commonly known as "marriage" in any way diminish, dilute, or degrade the "institution"? There is no answer.
I propose that we should "divorce" the body of civil laws that have grown up around marital unions from the sacrament called "marriage." Doing this, all persons who wished to join in a legal partnership would get their licence, or sign up at the registry office, homo- or-heterosexual. Those who wished could then have their union sanctified by their church, synagogue, temple, coven, meeting, or what have you as a matter of choice. If those same congregations chose NOT to sanctify the particular union in question for whatever reason, that would be matter of choice as well, but would not affect the legality of the partnership.
I am bemused by those who inevitably argue that "marriage is ordained by God," and the Bible. The same arguements of course were made in favor of slavery as an institution, and with as much justification.