While 2013 was a year of major victories for same-sex marriage in America, a controversial court ruling at the end of the year addressed another thorny marriage question: a federal judge in Utah struck down a portion of the state's polygamy ban. Many conservatives saw vindication of their fears that gay marriage would lead to legalized polygamy. In fact, the decision is limited in scope and does not actually change marriage law. But it also highlights a push for the acceptance of non-monogamous relationships that could change marriage -- and not for the better.

The ruling, which involves a fundamentalist Mormon man and his four "sister wives," does not require the state to validate multiple marriages: Legally, the husband only has one wife. It simply says such arrangements cannot be criminalized, particularly in a way that discriminates on the basis of religion (so that a man can live with several women and not face legal problems unless he calls them his wives in a spiritual marriage).

The Utah case represents a traditional, patriarchal version of polygamy. But there are also egalitarian, socially liberal subcultures in this country that embrace alternatives to monogamy: open marriage and polyamory (multiple intimate relationships with everyone's consent). These lifestyles have been gaining visibility, thanks in part to the discussions of same-sex marriage and new frontiers of tolerance.

Last August, Salon.com published an article titled "My Two Husbands," whose author -- living with her longtime husband and her boyfriend -- lamented widespread prejudice against families such as hers. A month later, Slate. com ran a pseudonymous piece by a man bemoaning the hardships of "the polyamory closet."

While the polyamorists often liken their cause to gay rights, the parallel fails in key ways.

First, in seeking marriage equality, gays could make a strong case that they simply wanted the same thing as heterosexuals. It would be much harder to make that argument for non-monogamous marriage. Sexual difference as a feature of marriage waned in importance due to the move away from strictly gender-based roles. But the idea of marriage as an exclusive union of two people is built into Western culture. (Indeed, monogamy was arguably the basis for the shift from patriarchal authoritarianism to affectionate partnership that eventually made same-sex marriage feasible.) The legal benefits of two-person marriage would be almost impossible to replicate with multiple partners.

The non-monogamists mostly want cultural acceptance rather than legal reform. But even that is fraught with problems.

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Same-sex marriage, as its proponents have pointed out, does not directly change or affect heterosexual marriage: It's extremely unlikely that a previously straight, married person will suddenly decide to get a divorce and marry someone of the same sex just because it's now legal. But a person in a monogamous marriage may well decide to renegotiate the "forsaking all others" part if non-monogamy gains cultural approval.

While polyamory and open marriage require mutual consent, first-person accounts often cast doubt on how freely given such consent may be. The Salon.com author acknowledged that her husband was hurt and angry when she decided she couldn't be "fulfilled" without extramarital liaisons. She claims that he eventually overcame jealousy and found their open relationship "liberating." Or maybe he simply didn't have the heart to face a breakup and a child custody battle.

If monogamy becomes merely one valid option -- a preference rather than a norm -- it will be much harder for people who want a sexually exclusive marriage to insist on one.

In a free society, multi-partner relationships should not be criminalized. But neither should they lose their stigma in the name of tolerance.

Cathy Young is a regular contributor to Reason magazine and the website RealClearPolitics.