My first criteria when it comes to the LAW is whether or not the LAW is constitutional in nature. The marriage laws as they current sit are an unconstitutional discrimination on the basis of gender in my mind, so I don't agree with them. I see no constitutional issue what so ever when it comes to discriminating against polygamists.
My next criteria would be is it a worth while, necessary, useful law that falls within some general scope of the Government. In terms of polygamy, I don't view it as exceedingly needed nor worth while in terms of the multitude of legal issues that it interjects into things.
I don't see any compelling reason to change the legal code significantly to allow for polygamy to be recognized under the law. "They're consenting adults" is not a compelling reason as we don't have the government officially recognizing MANY things consenting adults can do together. "But yo'ure letting the gays do it!" is not a compelling reason, because there's an actual strong constitutional reasoning behind allowing for that in my mind...one that is simply not present when it comes to polygamy.
No, polygamists shouldn't be able to get married in a sense that is legally recognized and incentivized by the Government.