If a man wishes to live with two or more women or, less likely, a woman wishes to live with two or more men, if all involved are consenting adults and there’s no reason to suspect coercion or that any children borne of the union are being mistreated, then I see no reason to prosecute them. However, I don’t believe that more than one spouse should have spousal rights as afforded by law (the legal next of kin status that is essentially what advocates of gay marriage are seeking) as this would cause way too many problems.
Using the Henrickson family of BIG LOVE as example- for those who don’t watch the show Bill Henrickson has three wives:
Barb- senior wife (the only legal one) with whom he has three children, married about 20 years
Nikki- the second wife, married about 10 years, 2 children
Margene- the third wife, married about 5 years, 3 children
Suppose Bill dies intestate. Should the estate be split 3 ways, equal amounts to each wife? This does not seem fair to Barb who was married to him longest and contributed more, nor does it seem fair to Margene who has 3 young children to support (Barb’s two oldest are almost grown).
or
— Bill receives major head injuries in a car accident and needs a high-risk major operation for which next-of-kin consent is needed. Barb says “do it”, Nikki says “don’t”, Margene says “I’ll go with what Barb and Nikki decide”. Who do you go with?
—Bill goes to work for a major company (on the show he’s self employed but this is “for instance”). Is it fair for the company to have to provide full medical to three wives and 8 children? (True, it’s the same number as if a monogamist had 10 children, but a monogamist isn’t likely to have 10 more kids in the next few years or have 3 women needing annual mammograms and other gynecological procedures.)
—Bill finally has it up to his eyeballs with Nikki and has Tony Soprano clip her. Margene sees the whole thing. Does Margene have to testify?
The biggest difference (of many) between gay marriage and polygamy is that gay marriage really doesn’t require rule rewrites. In the cases above, suppose that Bill’s gay cousin Will
—dies intestate; the estate goes undivided to his husband/partner/whatever you want to call it
—requires a high risk operation; the partner makes the decision
—goes to work for a major company; the company subsidizes insurance on the partner and any kids of the union
—goes on trial; his partner can claim exemption from testimony or, if he particularly chooses, may testify, same as a regular spouse
Polygamy however requires major re-writing of rights.