Yes, there really is a steep, slippery slope below Mt. Same-Sex Marriage

Kent Greenfield, who teaches law at Boston College Law School, points out an obvious point that is not obvious, from what I can tell, to a huge swath of people, including far too many political, cultural, and even religious leaders:

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. If we don’t want to go there, we
need to come up with distinctions that we have not yet articulated
well.

Greenfield continues:

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. Fundamental-rights analysis says that marriage is for
many a crucial element of human flourishing, or as the Court said almost fifty years ago
“essential to the orderly pursuit of happiness.” Because it’s so
important, government can restrict marriage only by showing a truly
compelling justification. The equal protection argument is simply that
the marriage right should not be taken away from groups unless the
government has good reasons to exclude those groups.

Put otherwise, it's not just that advocates of "same sex marriage" were aiming at a constantly moving (and nebulous) target named Equality, it's that the target is taking on a frightening life of its own—one that is logical, but built squarely upon flawed premises and assumptions. For example, as Greenfield notes,"prejudice—simply thinking something is 'icky'—doesn’t count as a reason" to not open up "marriage" to polygamists and incestuous couples. He writes:

The arguments supporters of same-sex marriage have made in court do
not sufficiently distinguish marriage for lesbians and gay men from
other possible claimants to the marriage right. If marriage is about the
ability to define one’s own family, what’s the argument against
allowing brothers and sisters (or first cousins) to wed? If liberty
protects, as Kennedy wrote ten years ago in Lawrence v. Texas, the case striking down Texas’s anti-sodomy law,
the “right to define one’s own concept of existence, of meaning, of the
universe, and of the mystery of human life,” why can’t people in
polyamorous relationships claim that right as well? If it’s wrong to
exclude groups because of prejudice, are we sure the uneasiness most of
us feel about those who love more than one, or love one of their own,
shouldn't count as prejudice?

All of this, of course, has been part of pro-marriage arguments for quite some time. Yet is almost always met with scoffing and derisive dismissal. One retort that I've heard many times is along these lines: "Oh, c'mon—you make it sound like this is inevitable, but it really isn't!" But that misses a key distinction: those, like myself, who recognize and believe that marriage is between a man and a woman, etc., aren't saying certain things (polygamous marriage, etc.) are inevitable, but that they logically follow once certain presumptions are accepted. Which is essentially what Greenfield admits:

In private conversations with leaders in the marriage movement, I often
hear two responses. The first is that there is no political energy
behind a fight for incestuous or polygamous marriages. The second is
that they would be fine if those restrictions fell as well but, in
effect, “don’t quote me on that.” The first of these responses, of
course, is a political response but not a legal one. The second is to
concede the point, with hopes that they won't have to come out of the
closet on the concession until more same-sex victories are won in
political and legal arenas.

He then attempts to articulate some "possible distinctions"; I'll leave it to you to judge their veracity. But Greenfield's concluding paragraph should give pause:

If these distinctions do not hold water, we have two options. 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.

Oh, that's reassuring. Very reassuring.