Do we know how this is going to play out, though? Has matrimony between same sex couples ever been widely available in any civilization? Rome, Greece, China and Egypt all had various different approaches to open homosexual relationships overtime, but it's hard to find any significant civilization that broadly equated same sex unions and heterosexual marriage. I'll allow that my research on this is incomplete.

I think anyone who thinks they know for certain how this kind of social change will play out on the scale of decades or centuries is mistaken. It's possible that everything turns out great and we enter a golden era of tolerance and flourishing human relationships, but at the same time there's usually something worth fearing in the unknown, which is why we tend to.