But it has always been between a man and a woman.
The article is claptrap.
It’s a bad headline.
Actually, there were long stretches of history in many places where marriage was not between a man: polygamy among the Jews, evidenced in the accounts of the patriarchs in the Old Testament (Jacob had two wives as did Lamech), was not abolished until the middle ages under pressure to conform to the Christian legal norm, then, of course, there is Islam which keeps up the Near Eastern custom of polygamy, places where Buddhists have practiced both polygyny and polyandry (different places), the Hindi record that in Vedic times polygamy was common (though Hindus are now monogamous), claims the Celtic pagans practiced polyandry, and a folk custom of polyandry in the Himalayas in which brothers share a wife.
Of course, except for oddities like Nero who “married” a eunuch and was mocked as a degenerate for it by his fellow pagans, it was always men marrying women, and involved a single spouse of one sex or the other.
Let’s see... Adam, Eve, and... well, no one else.
No “modern invention” to it.
What IS “modern” is that marriage has been separated from the function of procreating and raising children. And it is not even a NEW “modern” take on marriage. The Romans of the Imperial Age were well into the various forms of “non-traditional marriage” arrangements, all without the benefit of divine blessing. It was not uncommon for, say, a patrician Roman to marry his horse, or take multiple wives, or even an exclusive homosexual relationship. For the lower levels of society, trying to emulate these fanciful versions of “marriage” was just too expensive and rarely indulged in.
The pagan tribes within the Roman Empire, as a matter of course, did pair off as man and woman, as this was the most efficient economic unit that could be formed below the clan and tribal level. Had just about nothing to do with “divine” or “moral” standards. Or even “love”, for that matter.
I actually read the whole article on Cato’s web site. There were exceptions to the rule, but you are largely correct.