As the "traditional marriage" forces have been in retreat, both legally and rhetorically, there's an argument we haven't heard as much as we did a few years ago: that if you allow gay people to get married, then the same logic will demand that we also allow incest marriages and polygamous marriages. Today, Kent Greenfield grapples with it here at the Prospect; go read his piece, then come back and I'll tell you what I think about this.
My hunch is that the reason the incest argument has faded is that the anti-equality forces never gave it all that much thought in the first place. It was just something outside the prevailing definition of marriage that they thought would sound crazy to everyone, so they tossed it out there. The basic argument was that once you "change the definition of marriage," you'll be changing it to accommodate any preference anybody had. A man will marry his brother! A woman will marry her cat! A cat will marry a gerbil! (Bill O'Reilly is, for some reason, particularly troubled by the thought of interspecies marriage. Perhaps he doth protest too much?)
The reality is that we've changed the definition of marriage many times before when the definition was no longer in accord with our contemporary values (for instance, women who get married are now no longer their husband's property, and people of different races are allowed to get married), and one more change doesn't mean that there are no more limits whatsoever. As people became more comfortable with this particular change, the idea that it would necessitate other changes for which no one was advocating didn't have much persuasive power.
But more importantly, what the debate over marriage equality exposed is that the status quo definition of marriage never had much of a rationale behind it in the first place. It was just how we did things, and few people gave it much thought. When opponents of same-sex marriage were forced to define the rationale for the status quo, the best thing they could come up with was that marriage is only about procreation, a justification that falls apart on a moment's consideration (after all, we don't forbid postmenopausal women from marrying).
The debate also exposed that the anti-equality forces were completely unable to articulate a harm that could spring from gay people being allowed to marry. They offered some vague ideas about "devaluing" heterosexual marriages, but as the court in the Proposition 8 case found, there was nothing to them. In the end, since no one was able to show a demonstrable harm from gay marriages, no one was able to prove they had the legal standing to act as a party against such marriages, and that was where the case in favor of Prop. 8 fell apart.
That's where we come to incest and polygamy. As Greenfield describes, the case for the societal harm coming from incest and polygamy isn't all that strong. Even though many polygamous arrangements are terribly coercive, you can certainly conceive of ones that wouldn't be. If we wanted to, there might be a way to restructure the law to allow, say, three consenting adults who wanted to join in a union to do so, while still forbidding Warren Jeffs-style nightmares.
And yes, there's a nearly universal taboo against incest, and if forced to answer why that is, you'd probably respond that incestual relationships produce offspring with birth defects. How often would that actually happen? I doubt there's much data on the topic, since it's so rare. And what if a brother and sister in their 50s wanted to get married? It would be hard to say what harm would come from it. Yes, Joffrey Baratheon is a monster, but given the limitations of genetic analysis in Westeros, we don't know whether that's a result of his unusual parentage. And beyond the occasional tossing of a young boy out a window, who's really harmed by Jamie and Cersei's love?
To be clear, I'm not coming out in favor of incest and polygamy. But rolling these questions around, you begin to realize that it isn't something we've thought too much about. For the first time in our lifetimes, we're having an extended national debate on what marriage is for, as our own E.J. Graff put it. The answers can lead us to some uncomfortable places.