Frankly, polygamy is a far less serious challenge to the social structure, than making a mockery of family formation in relation to procreation. Solomon having multiple wives did not undermine the concept of marriage in the Old Testament. The present effort to divorce "marriage" from biological reality--i.e., who is able to procreate; why is humanity divided into two sexes, etc..--can surely undermine the social fabric.
Everyone reading this was once an adolescent. Did not most of those with whom you grew up, not have at least a major focus, in high school, on an effort to idealize a pursuit, or the invitation to a pursuit at the very minimum, for finding a mate from the opposite sex? The theme is basic, in some form, in most of the literature people read for recreation. It permeates the movies--even those produced by "artists," who themselves eschew traditional sex roles. The pursuit of sexual role adequacy & decent child rearing, inspire much of what ennobles individuals, through the generations.
The writer raises questions about allowing couples over what used to be referred to as "the age of hope," from marrying. But that is answered by the idea of giving anyone the benefit of the doubt--a benefit growing out of the report of the incredibly late in life birth to Sarah in Genesis. Of course, the ACLU might not like the concept, but earlier generations of the Western peoples considered the Bible a guide to what was morally acceptable. Like it or not, the concept of marriage has always been inextricably linked to family formation, via a sanctification of a properly mated--that is a naturally mated--couple's union.
Not only benefit of the doubt per the bible, but also establishment of a pattern. The couple is not bidden to divorce once they are past even medically proven childbirth years. And yes, Grandma and Grandpa can still “have sex.”
And getting backing for the bible means getting backing for God. It’s vain to complain that those who treat God as a passing acquaintance are doing a lousy job of sticking to the bible. The spiritual power to care about bible norms is not being had by those people and no, it cannot be ginned up on one’s lonesome.
From a worldly point of view, I would even say, if you think the bible is a great book to base a society on, then encourage every kind of evangelism you can in that society. “High minds” like Tom Jeff were only able to keep it up because there were so many Christian witnesses around them. Take those witnesses away and suddenly the “high minds” are wondering why they were bothering with that book.