Jesus himself reaffirmed the natural institution of marriage and brought it to completion in the sacrament. He both added the sacramental graces and also re-founded natural marriage in harmony with the Divine Law with which God began the human race (Mt 19:6; Mk 10:9).
If we read the Old Testament, we see that certain polygamous marriages were at least not explicitly condemned by God. (Implicitly, perhaps; or frowned upon; or judged unwise.) And divorce was permitted under certain circumstances.
Thus, God permitted the attenuation of the unity and indissolubility of natural marriage in his providential wisdom, as he labored to rescue the race that was going to ruin because of the Fall of Adam. However, Jesus brought the full truth and the grace whereby to live the truth. Thus, Jesus the Good Shepherd abrogated all such exceptions. Divorce and polygamy have no foundation in law any more, neither in the natural institution nor in the sacramental institution which began with Jesus.
Jesus’ decisions are decisions of Supreme Wisdom. For although polygamy is not per se evil abstractly considered, indissoluble monogamy remains the wisest order of things.
Monogamy. For how can one man figure out two women? And how could one man give himself totally to two women? The friendships would have to be halved. And how could a woman totally and enthusiastically give herself to a man who had another woman also? And if it is enough of a challenge to raise five children prudently with the collective prudence of one man and one woman, how could one raise ten children with two wives? And if it takes time to develop bonds and learn who one’s child is, then how much more difficult if children were being born every four or five months? And how would children see a total and undivided commitment without monogamous parents? Monogamy is thus the wisest order of things.
Indissolubility. The spouses need to know that they are sheltered unto death in this bond. They need to know that they are loved for their own sake, to the end. And children need to be raised in an environment that is stable, that can stand the storms of time. With an indissoluble bond established as the bedrock for individuals and also for society as a whole, spouses can live more and more freely, as they grow in the commitment that arises because of the bond. (Note: The commitment arises because of the bond. The bond is not the creation of the commitment.)
Jesus bequeathed to his Church the fullness of the Gospel whereby alone all these permanent decisions of Divine Law can be known by men and redound to the health of society.
His decisions have carried inertial momentum in society. However, societies have thrown off his Church and they tumble to ruin. For what Church remains that teaches the full truth on marriage? The Orthodox allow divorce and remarriage. Mainline Protestants allow divorce and remarriage.
What about other religions? Well, Islam would be a study for a different day. Mormonism allows polygamy. Secular societies in the wake of the French Revolution allow all manner of errors in connection with marriage.
Thus, increasingly, as societies have thrown off his Church, this inertial momentum, a momentum of life-giving and healing balm for society, has gradually slowed down. Divorce and civil unions were major battles waged against the truth of marriage. And now on the doorsteps is a most deformed notion of marriage, the proposal of homo-sexual marriage.
As it appears, insofar as societies fall away from Christ’s one true Church, they fall into these sundry errors.
The only group holding to the full truth in this matter is the Catholic Church, and the gates of hell shall not prevail against her.