"The Slippery Slope to Polygamy and Incest"

A law professor and supporter of same-sex marriage acknowledges that the slippery slope isn't all that scary. This is so on point.

You know those opponents of marriage equality who said government approval of same-sex marriage might erode bans on polygamous and incestuous marriages? They’re right. As a matter of constitutional rationale, there is indeed a slippery slope between recognizing same-sex marriages and allowing marriages among more than two people and between consenting adults who are related. 

The left is in this bind in part because our arguments for expanding the marriage right to same-sex couples have been so compelling. Marriage, we’ve said, is about defining one’s own family and consecrating a union based on love. We’ve voiced these arguments in constitutional terms, using claims arising from the doctrines of “fundamental rights” and equal protection. [...] Because it’s so important, government can restrict marriage only by showing a truly compelling justification.

[...] [The "ickiness"] argument goes something like this: “Well, gay marriage is one thing. But incest and polygamy are icky.” I understand this visceral response. But of course this is the same kind of repulsion that has been standing in the way of LGBTQ rights for decades, and which motivated anti-miscegenation statutes before that. This kind of argument makes us sound dangerously close to those who oppose same-sex marriage by claiming it is “unnatural.” 

[...] Incest raises the risk of birth defects, or so we’ve been told. But the risks are reportedly small, and probably less than for parents over forty, or smokers, or those with certain hereditary diseases. [...] This is the kind of thing we usually leave for people to decide for themselves. Here, too, the argument that marriage is about protecting the children sounds eerily familiar to the arguments trotted out against same-sex couples for years. And even if we wanted to intervene to protect the potential offspring of incestuous couples, there are things we could do (mandatory genetic counseling, for example) short of outright bans on their marriages.

[...] Perhaps polygamous and incestuous bonds are more likely to be coercive, especially for the women involved. Polygamy is often used to bolster a misogynistic, male-dominated family structure; incest is frequently the product or symptom of abuse and subjugation. [Wrong: likely a majority of familial sexual abuse is by siblings, and a minority of sibling sexual contact is non-consensual.] [...] And while polygamous marriages may more likely embody traditional stereotypes and roles, since when has that been a matter of government concern? As long as each individual who enters into a polyamorous relationship does so freely, and as long as divorce is available if they want out, then arguments from coercion are not particularly powerful.

[...] If the coercion argument doesn’t persuade, we could swing the other way and say sexual orientation is hard wired, but polygamy and incest are choices. [...] Maybe I am speaking out of school here, but arguments for marriage equality do not really depend on the claim that people have no choice about who they are. [...] Let’s be honest: If science revealed tomorrow that sexual orientation is fluid and changeable, the arguments in favor of marriage equality would essentially be the same, wouldn’t they? Just like our arguments for religious liberty do not depend on whether people are destined by biology to be a Methodist, our arguments about the liberty to marry need not depend on science.

[...] We can continue to search for differences that make sense as a matter of constitutional principle. Or we can fess up. We can admit our arguments in favor of marriage equality inexorably lead us to a broader battle in favor of allowing people to define their marriages, and their families, by their own lights.