a : the state of being married b : the mutual relation of husband and wife : WEDLOCK c : the institution whereby men and women are joined in a special kind of social and legal dependence for the purpose of founding and maintaining a family.
Any state legislature or court that wants to recognize "civil unions" among same-sex couples is perfectly free to do so. But anyone who presumes to call such a thing a "marriage" may as well issue an edict stating that 2+2 equals 38.7, or that the earth is flat.