What in the "Natural Law" provides this truth?
Religions just recognize that truth.
Not all religions share your opinion of this - some, very old ones provide for polygamy ...
Natural Law such as Man is different from Woman and when combined, they produce Babies.
I left out polygamy since it was a detail. Even in religions or cultures that allowed polygamy, it still mean Man and Woman or Women. And even in those cultures/societies/religions it wasn’t the norm. It served the purpose as well of providing husbands for women when many men were killed in battle, became monks or whatever.
Doesn’t hurt my argument aka “the truth” one tad.