Legalizing same-sex marriage does not simply extend an old institution to a new group of people. It changes the definition of marriage, reducing it primarily to an affectionate sexual relationship accompanied by a declaration of commitment. It then gives this more narrow view of marriage all of the cultural, legal and public support that marriage gained when its purpose was to encourage and temper a more complex set of goals and motivations.
Same-sex marriage changes the purpose of marriage law. It no longer will serve, in concert with other aspects of society, to direct sexual and parental behavior to achieve a complex synthesis of goods. It will function instead to extend marriage privileges to a particular group of sexual partners.