A very opinionated schoolboy recently told me that we ought to legalize same-sex marriage because marriage in the Old Testament was polygamous.
Why would legal polygamy three or four thousand years ago mean that same-sex marriage was valid now? I really cannot imagine, and it was not just his corkscrew logic that worried me (that is about par for the course in this sort of argument) but also the haste to accept a second or third-rate academic interpretation of the Bible.
What we find in the early part of the Old Testament is that Abraham and his successors were accustomed - and in some cases obliged - to take more than one wife. Jacob, the younger son of Isaac, wanted to marry Rachel and worked for her father Laban for seven years, but after the time was up he was tricked into marrying her sister Leah instead. (This was a sort of rough justice after the way Jacob had treated Esau, but that is another story.) Jacob had to work another seven years before Laban allowed him to marry Rachel. Both marriages were regarded as valid. Does this mean that marriage was polygamous? Not at all. Jacob's marriage to Leah united two people, a man and a woman. His marriage to Rachel also united two people, a man and a woman. Rachel and Leah did not marry each other!!! The word "polygamy" is often misunderstood. It does not mean just "having more than one wife". It means being permitted to enter into more than one marriage." Poly = many, gamy = marriage.
So with Jacob or anybody else, each marriage involved one man and one woman. Two was the maximum number of people involved in any marriage. And this is always true in any society regardless of whether or not they permit polygamy. It was not marriage that was polygamous, it was men who were polygamous if they married twice. It is still strictly accurate to say that each marriage was the union of one man to one woman, for life.
That is actually true of the institution of marriage in every time, and in every culture. Everything has an essence, and that is the essence of marriage. Those who try to prove otherwise usually offer a jumble of confused and wrongly-interpreted evidence, twisting facts to support their foregone conclusion. That's if you're lucky. They can just resort to denouncing anyone who disagrees.
"A little learning is a dangerous thing,
Drink deep or taste not of the Pierian Spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again."