Correct me if I am wrong but it appears that the Proposition 8 and DOMA cases are being argued by opponents on the basis of a supposed absolute right to marriage contained somewhere in the constitution of the US.
This is quite different from a challenge that could have been made on basis of the "normality" of such marriages.
If the argument that people have a right to marry whom they please without government interference is successful, it would seem inevitable that laws banning polygamy will be challenged.
What is the social benefit derived from laws against polygamy? As a libertarian constitutionalist I do not understand the laws against polygamy as being other than derived from Judeo-Christian tradition. pl