The problem comes when people confuse the civil law with God's Law. Look at divorce. It is a simple matter to get divorce in this country. Just show up at court with the papers signed and the man forks over all his money for the rest of his life, and the civil marriage is over.
But this does not and should not mean that the two people are no longer married. Unfortunately, for most folks, the civil law and moral law are equivalent, and the marriage is over when the court says it's over. Since our courts will always be permissive, since we will always vote to be allowed to do whatever we darn well please, having the courts decide on divorce can only weaken the institution of marriage.
Similarly with expanding the definition of marriage. Absent judicial intervention, the great mass of people will continue to think of marriage as the traditional joining of a man and a woman. A small minority may take a different view, but if there is no need for a judicial definition of marriage, then the view of the minority need not be imposed on the majority.
Thus if marriage truly is a durable societal archetype, it will continue as before, without governmental recognition or sanction. However, if government is involved, it can only destroy the institution.
Government can never create. It can only destroy.