Divorce was permitted in the Old Testament, but it was not approved by God.
Some of the patriarchs practiced polygamy in the Old Testament, but the practice died out while monogamy remained.
As for Adam and Eve, the first man and woman were declared to be husband and wife by God.
You have not named a single culture that practiced this “fertility testing” before their community considered them to be married.
Now, about fertility ~ did you see the trouble Abraham got into when he didn't "test first"? Yup. God had to provide him some direction to get an early heir!
That one event can be taken as a COMMANDMENT if you want. I am just guessing you don't want ~ but there it is.
Not to worry, I've met snakehandlers in my time ~ and they've told me that unless I take up the serpents I'm going to go to Hell, and you can't hardly beat that.
Still, the marriage to pregnant brides is hardly a myth ~ it's been going on among the peasantry for thousands of years.