Tom, I'd welcome some reason to think there's no link between the legal arguments in support of same-sex marriage and those supporting polygamy (along with some explanation of how that would relate to the criticism of Being staff, whose certainty has little to do with legal arguments).
As I understand the legal case, a central issue is whether there's any rationale for treating those who wish to marry someone of the same sex differently than those who wish to marry someone of the opposite sex. In the case of polygamy, the rationale for forbidding it even as a religious practice (which would normally be protected) is basically that traditional marriage must be protected, a rationale also advanced against same-sex marriage. If that rationale goes away, the most plausible basis for prohibiting polygamy does too. I don't see that as a problem, personally, but perhaps you do.
As I mentioned in what you replied to, Mouw doesn't argue that his religious beliefs should enjoy the force of law. His argument that same-sex marriage shouldn't be legal is a secular one.
More information about text formats