The Minnesota Legislature is considering proposing a constitutional amendment that would define marriage as the union of one man and one woman. Many opponents claim that definition is antiquated and discriminatory. A committed relationship should be the only criterion for marriage, they say.

But wait. What if a person loves two people, or three or more? If one man, one woman is a discriminatory limitation on the choice of a life partner, on what grounds can the state logically restrict marriage to two people? The fact is, once you adopt same-sex marriage - legally changing the standard for marriage from one man, one woman to a "committed relationship" - there is no principled way to prevent its extension to polygamy or other forms of "plural marriage" or partnership.

Outrageous scare-mongering! say the amendment's opponents. Oh, really?

Did you catch HBO's new prime-time series, "Big Love," which premiered Sunday? It's about a Utah man married to three wives.

The creators of "Big Love" are a gay couple, Mark Olsen and Will Scheffer, who say that the same-sex marriage debate spurred their interest in the topic.

They seek to normalize polygamy by treating it in a "nonjudgmental" way.

"Big Love" is just a TV show, you say? But cultural expression can pack a powerful wallop - witness the much ballyhooed bid by "Brokeback Mountain" to normalize same-sex attraction. Influential voices are already calling for allowing polygamy. Last week, New York Times libertarian columnist John Tierney endorsed its legalization in a column titled "Who's afraid of polygamy?"

Acceptance of polygamy might already be on the horizon in Canada, which recently recognized same-sex marriage. In January, a Canadian Justice Department report called for the decriminalization and regulation of polygamy, and warned the nation to prepare for a court challenge to two-person marriage. In a 2003 survey, 20 percent of Canadians said they are willing to accept polygamy.

Here in America, professors at elite law schools such as Yale and Columbia are laying the groundwork for legal recognition of committed relationships of three or more. Drawing on concepts borrowed from civil rights law, they say they aim to protect "sexual minorities" from discrimination.

Redefining marriage to include people of the same sex will open a Pandora's box. As a New Jersey judge wrote recently, if "marriage [is] ... couched only in terms of privacy, intimacy, and autonomy, then what nonarbitrary ground is there for denying the benefit to polygamous ... unions whose members claim the arrangement is necessary for their self-fulfillment?"

What's the likely endpoint? Marriage may be redefined and replaced by a flexible, contract-based system of government-registered relationships. So get ready. Today, gay-marriage supporters' mantra is, "How does my same-sex marriage harm your marriage?" Down the road it may be, "How does my marriage of two men and a woman harm your marriage?" If we don't answer the first question with resolve, we may not have a chance to answer the second.

Contact Kersten, a columnist for the Minneapolis Star Tribune, at kkersten@startribune.com.

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