It became customary around the time of Ezra. For that matter, many of the prophets before that were monogamous. Neither Isaac nor Moses were polygamous.
But it became a rule among the Jews about 1000 years after birth of Christ.
Minor point...
Polygyny - - one man many wives.
Polyandry - - one woman many husbands.
Polygamy - - any combination thereof, including homosexuality and pederasty.
The main issue (I do agree with the general arguments of the article) I see is the ongoing, perpetual struggle between the forces of Paganism and the Judaic.
This violent struggle from the very beginning of human history is manifested, only in part, by sexuality...
Pagan Babylon, pagan Egypt, pagan Greece, pagan Rome, pagan Islam, pagan National Socialism, pagan Marxism, pagan neo-Islam (once again), and modern neo-pagan New Agers.
Mosaic Law is the foundation of Western Civilization, it is something that neither the pagan Islamic thugs or the neo-pagan Left can abide and seek to destroy... they both seem to be on the same side, don't they?
It became customary around the time of Ezra says some - but it was the Christian church which introduced it - and the pagan Greeks and Romans were monogomous since recorded history.