The argument is not that it will "harm the institute of marriage" and it never has been. That is a gross mis-characterization of what the real complaint is. This has nothing to do with the number of divorces or how many unhappy marriages there are... What is harmed by allowing traditional marriage to be destroyed, is an irreversible erosion of the traditional family, and all that 'family' means to our culture and society. Gay Marriage undermines what many believe to be the cornerstone of a moral and decent society. Not to mention the fact that deeply religious people view marriage as sacred and a sanctified event with significant religious importance to the Church and the members of the Church. To make a mockery and sham of their religious traditions, is very possibly unconstitutional, but unethical and immoral at least.
The problem is easily fixed.... Civil Unions. Remove "marriages" from purview of the government completely, and do generic CU contracts, issued to any two consenting adults, regardless of what sort of relationship they have. Makes the religious folk happy, they can keep traditional marriage, makes the gay folk happy, they get all the marital benefits they wanted, it doesn't establish law based on sexuality or preferences, and it doesn't trash religion or religious expression. AND it protects the values of 'family' by keeping traditional marriages (ordained by the church only) intact.
But..... you'd rather argue pointlessly about this another few decades, I guess?