Have you ever even thought to consider that maybe there was once a really good reason why marriage was defined as a union between a man and a woman?
You know, all those thousands of years before antibiotics were invented in the 1950's, when childbirth was very dangerous, and surviving childhood was only a 4 in 5 shot?
Not to mention the hard-scrabble fact that families needed unpaid workers to work the farm to survive?
Civilization needed babies, and lots of them, to keep things moving along. Promoting a lifelong union between people who couldn't naturally have children just wouldn't even make sense in that world, since for most of human history, that was the very reason for marriage.
Now...flame on! I needed some downvote cred anyway...