Well, since you want to be all serious, I'll oblige.
And again, we run smack into the issue of basing laws on one group's faith
Due to a quasi-humorous comment from a non-Bible-thumping anarchist, we started talking about the Bible's position on polygamy. Nobody said there oughtta-be-a-law.
Telling me something should be legal/illegal based on what the Bible says carries as much water as telling me it should be legal/illegal based on the Chronicles of Narnia
Laws banning things should be based on harm being caused. Murder, rape, theft... all have obvious victims, so therefor are proscribed by law.
Tell me... exactly who is hurt if Spoon and I bring another woman into our relationship as a spouse?
I think it would enlighten the discussion to point out a couple of things.
While I agree with you on victimless crimes, there's no reason why a religious viewpoint should be less valid in politics than a secular one. I couldn't justify my belief in the rights of mankind, were there not a Bible to tell me that God created Man in His image.*
It is important to bear in mind that you are free (or should be free) to keep a [insert non-traditional arrangement here] household, whether your government recognizes your relationship(s), or not. FWIW, if we were to vote on polygamy, I think I'd probably abstain. I certainly couldn't vote for my government to recognize such relationships, but I don't know that I'd stand in the way, if the voters thought it should. It all comes back to the children of such unions, and the associated issues.
*And no, I'm not claiming that the Bible contains an Age of Enlightenment-style declaration of the rights of man. But I do find that modern (as opposed to post-modern) ideas about rights, libertarianism, etc, are the best way for a society to respect image-bearing Man in its politics. Because the Bible doesn't tell us what form of government we ought to have, if any, I must use my own judgment; as frightening as that may be.