If you are married to two women, can you claim them both as a dependent? Can all three of you file "jointly"? If one of those two women is married to a second man, can both husbands claim her as a dependant? Which claims her income, if there is any? If she has a child with the second man, but the child spends significantly more time with the household of the first man, can the first man claim the child as a dependant? What about if the second man is married to a completely seperate woman as well and they have a kid. The wife the two men share is now the "mother" of another woman's child, who and how can that child be claimed for tax purposes?
I have no issue with polygamy on a private level being legal. I don't believe that it's a benefit to society or government to legalize the notion in terms of the LEGAL term and benefits that come with it, nor do I find a compelling Equal Protection Clause argument for it in the same way there is for Same-Sex Marriage.