Marriage is society's best way of ensuring the well-being of children. State recognition of marriage protects children, we saw yesterday, by encouraging men and women to commit permanently and exclusively to each other and take responsibility for their children.

Laws on marriage work by promoting a true vision of the institution, making sense of marital norms as a coherent whole. Law affects culture. Culture affects beliefs. Beliefs affect actions. The law teaches, and it shapes the public understanding of what marriage is and what it demands of spouses.

But redefining marriage further distances marriage from the needs of children and denies the importance of mothers and fathers. Redefining marriage rejects as a matter of policy the ideal that children need a mother and a father.

The statistics on the importance of marriage penetrate American life to the extent that President Obama can refer to them as well known:

"We know the statistics -- that children who grow up without a father are five times more likely to live in poverty and commit crime; nine times more likely to drop out of schools and twenty times more likely to end up in prison," Obama said less than five months before he was elected president in 2008.

"They are more likely to have behavioral problems, or run away from home, or become teenage parents themselves. And the foundations of our community are weaker because of it," he added.

But how can the law teach that fathers are essential if it redefines marriage to make fathers optional? Redefining marriage diminishes the social pressures for husbands to remain with their wives and children, and for men and women to marry before having children.

Redefining marriage to include same-sex relationships makes marriage primarily about emotional union, more about adults' desires than children's needs.

If that's how we understand marriage, marital norms make no sense as a matter of principle. Why require an emotional union to be permanent? Or limited to two persons? Or sexually exclusive (as opposed to "open")?
Weakening marital norms and severing the connection of marriage from responsible procreation are admitted goals of the University of Calgary's Elizabeth Brake and other prominent advocates of redefining marriage.

Judith Stacey, a professor at NYU, has expressed hope that redefining marriage would give marriage "varied, creative and adaptive contours," leading some to "question the dyadic limitations of Western marriage and seek ... small group marriages."

More than 300 "LGBT and allied" scholars and advocates called in the statement "Beyond Same-Sex Marriage" for legally recognizing sexual relationships involving more than two partners.

In 2009, Newsweek reported that the United States already had over 500,000 polyamorous households. A 2012 article in New York Magazine introduced Americans to "throuple," a new term akin to "couple" but with three people.

Indeed, if justice demands redefining marriage to include the same-sex couple, how long before the courts demand redefining marriage to include throuples and quartets?

Some advocates of redefining marriage embrace the goal of weakening the institution of marriage in these very terms. Former President George W. Bush "is correct," writes Victoria Brownworth, "when he states that allowing same-sex couples to marry will weaken the institution of marriage. ... It most certainly will do so, and that will make marriage a far better concept than it previously has been."

It is no surprise that we see evidence of this occurring. A federal judge in Utah allowed a legal challenge to anti-bigamy laws. A bill allowing a child to have three legal parents last year passed both houses of the California state legislature.

If the law teaches a lie about marriage, it shouldn't surprise us when the consequences of that lie turn out to be bad -- for children and society as a whole.

-Ryan T. Anderson is the William E. Simon Fellow at The Heritage Foundation and co-author, with Sherif Girgis and Robert George, of the book "What is Marriage? Man and Woman: A Defense."