But before I begin on that, let me clarify my comment that I think of my mother's advice on "what if everybody did it?" to this question.
I follow this logical path.
Assuming that I have been discussing Chris, Pat and Lou who are members of my church and in a multi-partnered relationship, and beyond saying that they seem like nice people who are performing no harmful acts and good manners suggest that we not make a lot of judgments about things not our business, but go on to say, "Multi-partnered relationships are, in general, just as good as two person relationships."
To me, and permit me to think like a minister now, it follows that if Chris, Pat and Lou want to perform an unofficial "wedding" or "Union" ceremony, then I should perform it. And I am aware that when I perform an unofficial ceremony that looks like a wedding, I am making a public statement that I believe that the relationship in question should be, as a matter of public policy, able to receive marriage rights. I think that everyone understood that every same sex ceremony of union service is a symbolic call for equal marriage rights.
If you believe that multi-partnered relationships should be given equal marriage rights as two-person marriages, then it doesn't mean that you get to pick the composition and terms of those marriages beyond the minimal age restrictions we know place. My mothers question in my mind on this is "What if any group of three people want to get married?"
It means that the laws against polygamy must be abolished. How can UU triads be given marriage rights and not Mormans and Muslims? It means that marriages in which new and younger women are brought into the marriage on a serial basis, with the oldest wife's consent, must be legalized. It means that no standard that we would like to think of being essential -- careful negotiation, mutuality, egalitarianism, gender equality -- can be applied, unless we have the means to apply them equally to all marriages. It means, to be snarky, that we are in favor of multi-partnered relationships having equal marriage rights even if the people involved have never been in an OWL class.
Once I make the statement that multi-partnered relationships are in theory morally equivalent to 2 person relationships, I step beyond the pastoral (where my care for people does not presume approval of what they are doing in every aspect of their lives) into public ministry, where I am advocating public policy, where what I advocate will be operative not in some ideal world, but the world in which we actually live, with the human beings we actually see around us.
I have exactly the same problem with "assisted suicide". You say to me, "why can't it be legal for a member of my congregation -- well-educated, moral, loving, conscientious -- help his terribly suffering father take the pills that will end his life with dignity?" You offer a best case scenario. My mother asks "what if everybody could do that?" In this world, where we live, society does not protect elders from physical abuse and neglect in private homes and nursing homes around the country. And you want to give out the right to give fatal overdoses to their parents on an equal basis to all adult children of elderly parents? It is not enough to say that we give those rights only to people who follow the letter of the law about where and when and what conditions. We have laws against physical abuse and neglect of elders now -- we just can't enforce them, even in nursing homes which are publicy regulated, much less in private homes.
We live in a world where domestic violence, the coercion of women, the rape of underage female children by family members, sexual abuse of all types are occurring -- most of it stemming from the sense of entitlement given to husbands and fathers.
When you ask me as a minister to say, because it will make Chris and Pat and Lou, feel really welcomed and affirmed, that Multi-partnered relationships are just as socially beneficial and useful and moral as two person marriages, this is where I have to go in my thinking.