If stable relationships are so great, why not support gay marriage?
Social institutions that relate to fundamental things like sex succeed in channelling impulse reliably by integrating culture and nature, and can't be defined arbitrarily. They must be based on basic realities that don't change. The sexual bond between a man and a woman naturally gives rise to children, and therefore to a long-term connection that links the generations and transcends the immediate desires and interests of the parties. The institution of marriage strengthens, orders, and further defines that natural connection. It has accordingly been a fundamental social institution in all times and places, unlike "gay marriage," a modern eccentricity.
It follows that there is no reason at all to expect "gay marriage" to have anything like the institutional strength of normal marriage. Further, much of the strength of marriage has been its specificity, and its position in a network of related practices and understandings that define what sex is, what its place and importance in human life are, what men and women are, and what they owe each other. Recognition of "gay marriage" would separate marriage completely from such things, and so seriously weaken it as an institution while doing very little for homosexuals.