Two major figures from the religious right can be seen in the video below. Tamara Scott is Michele Bachmann's campaign chair in Iowa, and Bob Vander Plaats is a leader of the anti-gay movement in the state. Ms. Scott is worrying about how gay marriage will lead to all sorts of things, including women marrying the Eiffel Tower!
Here is a similar statement from Rebecca Kleefisch, Republican candidate for Lt. Governor in Wisconsin. She said, "This is a slippery slope. In addition to that, at what point are we going to be OK marrying inanimate objects. Can I marry this table or this, you know, clock? Can we marry dogs?"
Exactly how did they board this train of logic? It dawned on me today why they think such things are possible -- they have a wholly mystical view of marriage. You get this sort of nonsense from most Republicans. For example, Ron Paul has said that marriage should be "returned" to the church. Why the church?
For centuries, marriage was a civil affair, the legal equivalent of a contract today; it was not particularly religious. While legal systems of that day were not nearly as complex and pervasive as they are now, marriage was a contractual matter, not a spiritual one. It took several centuries of the Vatican fighting civil society in order to take control of marriage away from individuals. In The Transformation of Family Law, Mary Ann Glendon writes, "The greatest obstacle to the direct enforcement by the Church of the new Christian ideas about sex and marriage was that marriage was regarded everywhere in Europe in the first half of the Middle Ages as a personal and purely secular matter."
Since this was the case, even churches were prone to recognize privately contracted marriages, without the use of a priest or church in the process. It took a few more centuries for that idea to be mostly eradicated, though it still survives in some segments of the legal system.
The power grab by the Vatican over marriage meant a lot of people associated marriage with mystical ceremonies performed by Christian shamans, invoking blessings of a deity on the couple, but it was the contract between the couple that made them married, not the ceremony.
Right from the start, the American colonies saw marriage as a civil contract, yet the United States allowed religious figures to act as agents of the state, or the legal system. Many marriages still have mystical elements associated with the ceremony, but what makes the marriage legally binding is not mysticism but the contract. When a minister says, "By the powers invested in me by the state of ... I now pronounce you man and wife," that is when the marriage becomes binding. The minister acts as a witness to the legal contract of marriage, and that makes it a legal union of two people.
Marriages without the mystical elements are still legally binding. Religious ceremonies, prayers, candles, priests and all the trappings of modern religion have nothing to do with making a marriage contract real. It is the legal consent of the two partners, not the ceremony, that makes a marriage valid.
If you take the contractual view of marriage, as was prior to the church takeover of marriage, and as it has been since the Age of Enlightenment and the rise of classical liberalism, you immediately see the absurdity of the concerns of people such as Scott and Kleefisch. How does one enter into a contract with a table, a dog, or even the Eiffel Tower? Contracts are between adults who have the ability to understand their contract and are capable of consent. A dog can't understand a contract, nor can it consent. Neither can an inanimate object or a child. You can't marry someone who is comatose, for the same reason. And while Mormons may baptize for the dead, they haven't tried marrying them off.
The nature of a contract requires capable, consenting individuals voluntarily entering into an agreement. Without them, no agreement, thus no contract is possible. Marrying a table is legally impossible, and not even the Eiffel Tower can sign a contract. I doubt even the best-trained dog would be able to do so, either. This is why we don't allow contracts with animals, objects, or children.
However, if you believe marriage is some form of mysticism that exists because a shaman said the right ceremonial words, invoked the right deity, or read from the right sacred writings, then such ceremonies are possible. In that view, it is the magic that creates the marriage, not consent. And a priest, on the other hand, can invoke mystical claims in order to marry a man to a rock, or a woman to the Eiffel Tower.
In the end, this means their criticism of this absurd slippery slope does not, and cannot, apply to the contractual, legal nature of marriage, as has been the normal course of human affairs. Their slippery slope could exist if you believe that incantations, candles, shamans and deities create the marriage through a sacrament or ceremony. Their critique doesn't apply to the secular view of marriage, but it should raise concerns about those who take marriage out of the realm of legal contracts and want to give it back to "religion," where they assume it belongs. In psychological terms, their concern is called projection.