I could really care less about polygamy - if consenting adults who care for each other are willing to form a "household" and accept the legal obligations to eachother, then so be it. If their religious authority wishes to spiritually bless the affair, then all the best to them; it is none of my concern.
As far as I understand it, the "legal" definition of marriage concerns the relationship of the married people to the state. The "spiritual" defintion of marriage concerns the relationship of the married people to a higher plane/power; and that is up for them and their religion to decide. As far as the legal definition goes, all I am really concerned about is consent (thus eliminating animals, children, and inanimate objects), no criminal code violations (I think we can stick incest here), and a legal framework of obligations and entitlements.
Think about it, other than that, there are no "hard lines" that we can legally hold married people to; consider all these "man and woman" things:
- Love (prearranged marriages certainly aren't based on this, although it is hoped that it will later form)
- Children (many couples get married and choose not to have children)
- Tradition ("man and woman" is not the only "tradition"; certainly in our past somewhere, men have taken on multiple wives, while in many ancient societies, marrying within the family was "traditionally acceptable" and preferred)