One might assume that anti-bigamy laws would rule out plural marriages, but I don't think it's necessarily so, at least these days. Numerous religious groups--the Unitarian-Universalists, for instance--perform marriage ceremonies for same-sex couples, and although most states don't recognize these unions, it's not illegal to enter into them. What would be different about polygynous or polyandrous marriages, outside of those (few?) small towns in which it is illegal for unmarried couples to cohabit? I don't know of anyone being prosecuted for adultery recently, even where those laws are still on the books, and a married man who fathers a child with a woman not his wife is still, as far as I know, liable for child support.

Why couldn't a couple, Muslim or otherwise, marry legally, then add a second wife to the marriage in a religious, civilly non-binding ceremony, and arrange among themselves through contractual means (like those employed by some same-sex couples) to safeguard her interests and those of any children she may subsequently have? As long as the second marriage doesn't purport to be a civil marriage as well as a religious one--and as long as the applicable state constitution doesn't contain any amendments intended to prevent any such legal maneuvers--wouldn't a polygynous or polyandrous marriage be legal in most jurisdictions, at least in the sense that no one would go to jail for being in one?

"Why couldn't a couple, Muslim or otherwise, marry legally, then add a second wife to the marriage in a religious, civilly non-binding ceremony, and arrange among themselves through contractual means (like those employed by some same-sex couples) to safeguard her interests and those of any children she may subsequently have?"

>>Because those private legal contracts are not sufficient to vest the second wife all the benefits, protections, obligations, and rights the first wife received automatically upon recognition as a party to a civil marriage, anymore than they're sufficient to vest same sex couples with those benefits. The second wife could never have an equal standing before the law with respect to teh first wife.

That is why the usual practice amongst the local Fundamentalist LDS polys is to marry, and then divorce each wife -- gives some legal standing to each (technically) ex-wife.

Of course, most of these wives have fuck-all for rights anyway, as a practical matter, so it is fairly academic.

Cohabitation is actually illegal in a few states, not just a few towns. I live in North Carolina and I know it's illegal here, though it's very rarely enforced.