American Anthropological Association

Of course, they're just a bunch of lefty academics, but here's what they have to say on the subject of marriage

The results of more than a century of anthropological research on households, kinship relationships, and families, across cultures and through time, provide no support whatsoever for the view that either civilization or viable social orders depend upon marriage as an exclusively heterosexual institution. Rather, anthropological research supports the conclusion that a vast array of family types, including families built upon same-sex partnerships, can contribute to stable and humane societies.

The Executive Board of the American Anthropological Association strongly opposes a constitutional amendment limiting marriage to heterosexual couples.

Wouldn't it be nice if more people had some knowledge of history and related disciplines?