This article from the WaPo clearly lays out why the supposed threat to “traditional” marriage posed by same-sex marriage is based on a misunderstanding:
We are near the end of a two-stage revolution in the social understanding and legal definition of marriage. This revolution has overturned the most traditional functions of the institution: to reinforce differences in wealth and power and to establish distinct and unequal roles for men and women under the law.
Over time, marriage largely ceased to have safeguarding property or making alliances between powerful families as its primary function. Instead, people came to believe that they should marry for love and happiness. The feminist revolution followed, and we gradually came to understand that marriage didn’t have to mean prescribed roles for men and women either. Modern, heterosexual companionate marriage is, therefore, “traditional” only in a very attenuated sense. The most radical changes are already behind us; gay marriage is simply one more step in the institution’s evolution.