Well, IMO, the family, which is based on heterosexual marriage, is really the cornerstone of society, always has been, in any society. That is why heterosexual marriage has always, throughout the history of man, enjoyed a special status.
Granting the same status to homosexual marriage may damage the special status of traditional marriage, since it's suddenly no longer quite as special, and in turn, damage the family, and therefore, society.
Is this institution, this veritable cornerstone of society, so fragile that it cannot survive without governmental protection?
Get government out of the marriage business, before it is too late!