For once I agree with you on something! I don’t remember polygamy being a states rights issue in the 19th century. The Federal government imposed its will on the West on this and a host of other issues.
It is also worth noting, the homosexual advocats are using the legal argument of polygamy vs “just two”.
They are using the “winning with words” guru Professor Lackoff’s spin of saying “its about two consenting adults wanting to live together in love”. (IOW Marriage does not include children BTW)
Marriage has also been a federal issue in immigration. While other nations allow for polygamy AND homosexual marriage, long before the 1996 DMA the immigration service (INS then and USCIS now) did not allow such federal recognition. That was mere federal regulation!