It's interesting to note that even among cultures that accepted homosexuality, like the Graeco-Roman world, there was no homosexual marriage. ("Homosexual" is Latin, but the ancient Romans didn't even have a word for it!)

Also, in reference to incestuous relationships, cultures that have accepted that (such as the nobility in Pharaonic Egypt), usually did it to keep power in the family. Of course, I've always felt the entire institution of marriage was a product of agriculture, it was necessary to have a system to ensure the orderly passing of the farm to the heirs. Love has had little to do with it, historically. It was families that cemented alliances and relationships with marriage, the participants had little to say about it.