Nope. My concern about Adam’s post is that he didn’t have legitimate (or even clearly stated) concerns, but rather that he engaged in a bit of illogical guilt by associationism rather than present a clear argument.

Josua: “Generally speaking there are good scientific reasons to think that men tend more toward polygamy than women. Indeed, the woman with many husbands in history is rare, and women are more committed to their partners in general.”

But this doesn’t mean that polygamy favors men. It means that polygamy (understood in a free-choice way and not forced-harem way) favors:
(1) attractive men
(2) women who see their choice as between sharing an attractive husbands or having an unattractive ones to themselves, and prefer the former

and disfavors:
(1) unattractive men
(2) women who would otherwise have a shot at the most attractive men on the marriage market to themselves

So yeah, these considerations matter, but they’re not as straightforward as you suggest. And more importantly, while they may be reason to keep civil marriage monogamous, they’re not enough for me to be nasty to acquaintances in non-traditional relationships.

However, I’m sure there are plenty of things that are both legal and favored by one group more than another. This doesn’t seem like sufficient legal, or even moral, reason to resist its legitimization.

So who knows.

We atheists are great at meta-ethics, but sometimes not so good at ethics. We’ll insist that, in fact, anything doesn’t go, but are hard pressed to put our fingers on anything that doesn’t. It would seem we’re prone to shortsightedness. We’ll condemn a practice that does immediate, noticeable harm. But if it’s something that takes a while for the bad consequences to develop, we don’t notice and often we’ll kid ourselves that there are none. That allows us to indulge ourselves and then to protest that “correlation isn’t causation” when the contrary results finally descend on us.

Yes, I know religion often forces moral precepts on us that don’t stand scrutiny under the harsh light of reality. On the other hand, religion sometimes scrapes up a bit of wisdom as it trundles along for centuries of dealing with the vagaries of the human constitution. It might be best not to be like children leaving home for the first time and finding out the hard way that not everything that can be done ought to be done.