To go back to Endyss' earlier point, in asking for justification as to why we shouldn't be open to polygamy if we're going in to re-write and re-interpret marriage laws:

What I like best about law is the "Average Person" identification. What would the average person do? What does the average person think?

The average person believes that two people should be together in eternal companionship, or at least till the end of life. Since before there was a notion of God as defined by Christian religion, since before most religions today were even around save the pagans and wiccans, humans have naturally flocked to the idea of two people. That's just how love works, baby.

We would not protect polygamy in our new laws because some crazy sects of people practice it just as much as we would not protect public nudity in our laws because some awesome sects of people practice it. The average person hasn't changed their mind on that. Not at all.