One typical objection that I hear from opponents of gay marriage is that expanding the definition of marriage is a slippery slope. We need to stay true to it’s original definition. Otherwise, we’re opening the door for marriage to be further eroded. If we allow same-sex couples to marry, what’s keeping us from allowing people to marry multiple people? Or their dog? Or their car? No. Marriage has always been between one man and one woman. It should stay that way.
For the moment, I’ll disregard several of the blatant logical fallacies in the argument above (like the fact that dogs and cars have no legal standing and can’t give informed consent).
The foundation of this argument is that we should redefine marriage according to its historical meaning. Deviating from this definition would be a slippery slope. But why does the slippery slope always go to the left?
Until recently, marriage was defined as between a white man and a white woman. Interracial marriages weren’t legally recognized until 1967. So if we’re going to stay true to the historical definition of marriage in this country, we should make it illegal for white people to marry black people (or people of color to marry at all).
If we want to go further back in history, we could define marriage as a business arrangement for the transfer of property. The groom paid the bride’s family a dowry in order to purchase her, since there are many economic advantages to owning someone who had no official legal standing.
Also during this time, most marriages were arranged–sometimes years or decades in advance. Often, neither the groom nor the bride (but especially the bride) had any say in the matter. Perhaps this is the model that we should revert to. This is, after all, a more historically consistent definition.
My point is that the slippery slope doesn’t always go to the left. Furthermore, I doubt very much that anyone would argue against the changes made to the institution of marriage over the past several centuries. I would guess that most people would consider these moral progress.