I will agree that marriage was created for social and economic reasons but not the biological aspect. When women were once seen as chattel, marriage had many economic implications, procreation was only one of those. Marriage ceremonies didn't apply to more than 90% of the populace for most of history. It was only the elite, the nobility, the One Percenters who used marriage ceremonies to unite families, tribes, clans and nations. In some cultures, the joining of lovers had many permutations. Unlike what we think of as the mediaeval standard, there are recorded joinings in matrimony of two male nobles with the sanction of the Church. In Asia, it is known that some groups used polyandry to maintain their culture - men died too frequently, so women often had more than one husband at the same time. Native American tribes were known to accept the "two-spirited" as natural beings.
So basically, the whole, "marriage has always been one man and one woman" is historical nonsense.
