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.

It’s been a few weeks since the victories in the marriage cases at the Supreme Court, and maybe it’s time for the political left to own up to something.

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.

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.

What it boils down to is that when the government wants to exclude groups from something important like marriage, it has to show good reasons for the exclusion.

When it comes to marriage, the fundamental rights claims and the equal protection arguments often intertwine. For example, Justice Kennedy’s opinion last month striking down a portion of the Defense of Marriage Act said that DOMA’s injection of “inequality into the United States Code” violated the “liberty” protected by the Constitution. The “inequality” part is equal protection language; the “liberty” wording is fundamental rights stuff. The analytical box is not all that important. What it boils down to is that when the government wants to exclude groups from something important like marriage, it has to show good reasons for the exclusion. And prejudice—simply thinking something is “icky”—doesn’t count as a reason.

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.
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