The DesNews had an article regarding gay marriage and a possible external result of the gay marriage battle. If gay marriage is ultimately upheld, as I suspect it will, other seemingly reprehensible exceptions such as polygamy or polyandry will have to be allowed also despite arguements made by gay & lesbian rights coalitions that there aren't similarities between the two issues. However for tax reasons polygamist stand to lose substantial amounts of tax refund money if polygamy becomes a recognized and defined form of marriage.
For example, you have one polyg man and his original wife filing joint with income of 30k with 3 kids, he has a tiny amount of tax, all which would be swallowed by child tax credits, any remaining child tax credit and earned earned income credit (welfare for the working poor) is refunded to him. 2 to 3 thousand dollars in money the family never paid in -- free money. Then you have wives 1,2,3 .... etc each with 2 to 3 children from polyg man. The extra wives likely make around 15k to 20k per woman thereby maximizing the earned income credit for total family refunds of 11k to 20k in free government money. If the polygamist family group had to file taxes on only one return, all those refundable credits we be limited and diminished as the extra wives income is mingled with married couple number 1's income in one return.
Polygamist make alot of money (your tax dollars) off of their oppressed circumstances. I see a fair number of polygamist and they usually walk out of my office with refunds of 3-6 thousand dollars in refunds per return most of which is refundable tax credits.