Either way, there is a point to asking how one could make an argument in favor of SSM and not polygamy. I suppose it could be done on the argument that SSM is still one person married to one other person, and that polygamy would bring with it a wholly different legal challenge in terms of finding standard ways to define its contractual terms. E.g., that a “liberal” form of polygamy would somehow have to acknowledge the equality of all partners to the marriage contract, and that this might prove difficult given the extent to which the vast majority of contractual uses of marriage at the moment (governing inheritance, joint assets, custody and raising of children) and so on are extremely geared to two-person relations. This would be a practical rather than philosophical objection, though it might have larger philosophical underpinning (e.g., that there is something about a three-person relationship that is intrinsically more unstable).

One could extend from this to observe that SSM marriages largely operate under the same general cultural understanding of marriage and relationship as heterosexual marriage, that they’re both about the dominance of the companionate ideology that arose in the early 20th Century, and that polygamy, whether in the Mormon form or in some 21st Century form, would not necessarily come in under that paradigm–that we don’t really have any cultural language beyond the Biblical, the non-Western, or polyamorous. to imagine polygamous *marriages*, all of which depart from the companionate ideology (SSM or otherwise) that structures our current understanding of marriage.

But I do think this is a legitimate issue to raise (as opposed to the truly misbegotten attempts by some critics of SSM to raise pedophilia, bestiality, etc., which really are different cases). If we saw more and more stable, “companionate”, three-person households arising where the practictioners started demanding marriage instruments, then I think you almost couldn’t avoid taking that request seriously.

1) Once gay marriage is legalized/formalized, there will be calls for legalizing polygamy.

2) The test cases for this will feature Muslim legal immigrants to Canada and US states like Massachusetts with polygamous marriages entered into in their home countries. These families will feature as much love/affection as any heterosexual or homosexual couple could ask for.

3) The plaintives will be able to make arguments at least as good as the proponents of gay marriage (with regard to history, justice and the like).

4) Once it is accepted that there is no reason to restrict marriage to a man/woman pair, it seems unreasonable to restrict it to a two person pair.

5) Polygamy will be made legal in at least some areas.

6) Many wealthy powerful men will discover that polygamy is a wonderful thing.

7) Society will be worse off.

I am not asserting that I agree with this, but it certainly seems like the strongest case that the anti-gay-marriage crowd can make.