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.
Though not as common as marrying a figment of one's imagination.
It's amusing how many of these arguments for same-sex marriage are so, ….uh...twisted.