The idea of Natural Law came from Thomas Hobbes, and later from John Locke. Ben Franklin was also influenced by that school of philosophy.
They took it at least partially from Orthodox Jewish practice (with which I am not familiar enough to cite the applicable laws) and partly from Christ's amplification of the Jewish Law (Torah.)
The question I ask, is where in the Gospels is Christ attributed to having spoken on the issue of monogamy or polygyny?
We know it was explicitly permitted by Jewish Law (which is from Moses) to have polygyny and not polyandry.
Whenever I have questions about Biblical law, I refer to Hobbes.
Part IV. Of the Kingdom of Darkness
Chap. xlv. Of Demonology and other Relics of the Religion of the Gentiles.[10] Another relic of Gentilism is the worship of images, neither instituted by Moses in the Old, nor by Christ in the New Testament; nor yet brought in from the Gentiles; but left amongst them, after they had given their names to Christ. Before our Saviour preached, it was the general religion of the Gentiles to worship for gods those appearances that remain in the brain from the impression of external bodies upon the organs of their senses, which are commonly called ideas, idols, phantasms, conceits, as being representations of those external bodies which cause them, and have nothing in them of reality, no more than there is in the things that seem to stand before us in a dream. And this is the reason why St. Paul says, "We know that an idol is nothing": not that he thought that an image of metal, stone, or wood was nothing; but that the thing which they honored or feared in the image, and held for a god, was a mere figment, without place, habitation, motion, or existence, but in the motions of the brain. And the worship of these with divine honour is that which is in the Scripture called idolatry, and rebellion against God. For God being King of the Jews, and His lieutenant being first Moses, and afterward the high priest, if the people had been permitted to worship and pray to images (which are representations of their own fancies), they had had no further dependence on the true God, of whom there can be no similitude; nor on His prime ministers, Moses and the high priests; but every man had governed himself according to his own appetite, to the utter eversion of the Commonwealth, and their own destruction for want of union. And therefore the first law of God was: they should not take for gods, alienos deos, that is, the gods of other nations, but that only true God, who vouchsafed to commune with Moses, and by him to give them laws and directions for their peace, and for their salvation from their enemies...
Although monogamy was the Christian norm from Apostolic times, concubinage seems to have been tolerated well into the Christian era, though clergy were forbidden to take concubines even as they were forbidden to remarry. These fact, together with the fact that we have no record of Our Lord speaking against the polygamy of the patriarchs makes me wonder whether monogamy is really part of universal moral law, or whether it may---unlike non-adultery---be an ascetic discipline incumbent upon Christians in particular rather than on human beings in general. If it is a Christian ascetic discipline, insisting on the state enforcing it on society in general is rather haughty and intolerant. I don't, for instance, want the government to enforce the keeping of Great Lent according to the Orthodox rule, while I do expect my spiritual father to give me an epitemia if I break the fast.