Apart from any of these other objections, polygamist unions seem likely to prove less stable than two-person unions, which aren’t particularly stable themselves these days. If each individual in a polygamous union is no more or less likely to seek a divorce than a person in a monogamous union, the failure rate would still be at least a third higher, assuming a three-person grouping, and higher still for larger plural marriages. That isn’t sufficient reason to punish people for attempting polyamorous unions, but seems like a good reason to avoid encouraging them.
The option of plural marriage might also destabilize some two person unions, with one spouse regarding the existing arrangement as “till death do us part,” only to be confronted with a spouse who, while averse to divorce, is pushing for a new member of the marriage. “Either she joins us,” a husband might say, “or I’m out.” It’s hard to say if changing norms would make that scenario more likely than it is now.
Then there are the logistical problems that plural marriage presents, which would seem to require altering core features and benefits that presently make up civil marriage. Mary Anne Case, a law professor at the University of Chicago, has pointed out that the legal institution is largely concerned with the “designation, without elaborate contracting, of a single other person third parties can look to in a variety of legal contexts.” Three, four, or five person unions would require abandoning that aspect of marriage.