One thing that I found very interesting during the marriage equality debate was the concept of traditional marriage. It made me do a bit of reading on the history of marriage - traditional marriage is NOT what a lot of opponents made it out to be. It certainly did not start out as a religious ceremony - it was co-opted by the Catholic Church (well, at least in Europe) around the 13th century. And you'd better be eyeing up your cousin if you want your marriage to be traditional.
History of Marriage: 13 Surprising Facts
Though marriage is now seen as love between equals, throughout history it has had a radically different definition
www.livescience.com