I don’t think “it may be indirectly destabilizing to society” should be our criteria for manipulating the state to make something illegal. One could argue that it’s inherently harmful to children in a more direct sense, and should be outlawed on those grounds, but that somewhat harder to argue given the biblical examples, and the fact that while God chose to specifically disallow all manner of sexual practices, he declined to forbid polygamy.

Given that we also have the New Testament, I don’t advocate Christian churches or pastors giving their blessing to polygamous marriages (just like I don’t think they should give their blessing to homosexual marriages), but I don’t necessarily see a problem with the secular state allowing consenting men and women to enter into polygamous “civil unions”.

Honestly, I doubt they would be very common. Historically speaking, women entered into polygamous unions because they had no choice. From the man’s perspective, having multiple wives was easier to “manage” because he enjoyed a position of near-absolute authority over them. Given that women enjoy some measure of equality today, I can’t see many of them choosing polygamy over monogamy. And from the man’s perspective, I don’t see many men signing on for the “hassle” (not to mention financial obligation) of having multiple wives.

If a man wants to have sex with more than one woman, the “easiest” way is to enter an ostensibly mongamous relationship with one, then cheat. That’s much easier than finding two women willing to enter into an “openly” polygamous relationship.