Reasoning with Adam and Steve

I got to reflecting recently on the effort to legalize gay marriage, and how Christians might contribute arguments that pack some punch with people who do not accept biblical authority.

One argument worth making is what I would call (for lack of a better descriptor) the witness of thousands of distinct cultures over thousands of years of written and/or oral history. Can anyone identify a few cultures or historical periods in which same-sex relationships resembling marriage were known to enjoy legitimate status?

Such example would of course need to meet a few standards of evidence. Confirmation from more than one source, for instance, that such practice did in fact exist and that the period in question lasted a historically significant amount of time (i.e. Mayor Gavin Newsom's short-lived San Francisco experiment doesn't count).

I, of course, really doubt that this precedent can be found, and that's my point. Is it not with breathtaking hubris that our current Western civilization (certain segments of it, anyway) dismisses the witness of the entire history of human civilization to promote this novelty? That with regard to a fundamental issue like what constitutes a family, we have nothing to learn from the collective wisdom of every generation and culture from our grandparents' back to the dawn of time?