What's wrong with consensual polygamy? What's wrong with siblings wanting to marry each other, especially non-blood siblings?
My two cents as a Christian is that it is not the government's place to enforce religious teachings as law. A government that enforces Christian teachings is no different than a government that enforces Islamic teachings (other than the base religion, of course). The notion that government should enforce religious teachings goes against the very foundations that make this country great.
Our system of government cant handle extreme polygamy. ...I could marry every single person in the US quit my job, get on food stamps and welfare and make all the rest pay for it. Imagine it....One could marry every single person on the planet...theyd all become citizens with absentee ballots. EBT cards and wic.