"How would same-sex marriage hurt your marriage?" Advocates of changing our marriage laws tell us this is an unanswerable question.

A typical couple -- Mary and John, married for 15 years -- may find it tough to answer. That's because it's the wrong question. Mary and John won't stop loving each other or be bounced out of their house if same-sex marriage prevails. To get at what's really at stake, we need a different question: "How will same-sex marriage harm the institution of marriage -- and in the long run, all of us?"

Marriage is a universal human institution. Across the world and throughout history, it's been exclusively male-female. That's not because of antigay bigotry, but because marriage is anchored in a primal biological and social fact: Sex between men and women creates new human beings.

The primary purpose of marriage is to ensure the best environment for rearing the children born of male-female sexual acts. Marriage channels men's and women's sexual attraction into productive ends, and harnesses the male sex drive by binding men to the mothers of their children. The evidence is overwhelming: Boys and girls flourish best with a married mother and father, who perform different and complementary roles in preparing them to deal with the world and the opposite sex.

Same-sex marriage would not -- as advocates claim -- merely extend the benefits of marriage to more people. It would gut marriage of its fundamental meaning and transform it from an institution centered on children and the mother/father nuclear family to one centered on adults. Marriage would become an artificial institution, bestowing state approval on any adult relationship based on affection and interdependence.

Such a redefinition would compel us to repudiate time-honored ideas of social organization. Last year, in mandating gay marriage, the Iowa Supreme Court rejected the belief that children need a mother and father as a mere "stereotype." Courts are also beginning to upend our ideas about parenthood -- jettisoning biological ties, recognizing "psychological" parents and including three-parent arrangements, with unpredictable results.

Same-sex marriage may not change the lives of John and Mary. But their children and grandchildren will bear the brunt of this cultural revolution. Today, only 59 percent of children live with their married biological or adoptive mother and father -- a result of divorce, cohabitation and rising out-of-wedlock births. If same-sex marriage prevails, the marriage culture is likely to erode further.

In European countries and American states where same-sex marriage is legal, the proportion of gays choosing to marry is well below that of the heterosexual population. In America, about two-thirds of gay couples who seek legal recognition are lesbians. The larger society does not expect or pressure gay people to marry -- for them, it's just a matter of personal preference.

Over time, this attitude could reshape the larger institution of marriage. As social norms that have encouraged men and women to take on the hard work of raising a family unravel, heterosexual couples are less likely to see marriage as important or relevant. Increasingly, marriage is likely to become just one of many options in a lifestyle smorgasbord.

If marriage is primarily about children, some ask, what about infertile and older couples? If infertile male-female couples do adopt or have a child, that child will have a mother and father. The human body's design makes clear that men and women -- whatever their age -- are naturally directed toward each other and complement one another.

If same sex-marriage prevails, we are likely to see further attempts to "expand" marriage.

Once marriage is stripped of its organic purpose, why restrict it to two people? Two lesbians and the sperm donor for their child, polygamists, bisexuals: All will want society to recognize and respect their relationships.

And why should marriage be open only to people with a sexual relationship? That discriminates against two female friends who want to share the burdens of rearing their kids, or a disabled brother and sister who live together. Some of the most influential proponents of same-sex marriage seek to "get the state out of the marriage business" altogether.

It's ironic that in other realms of life, Americans are very aware of the risks of tampering excessively with nature. Many of those urging us to transform humankind's fundamental social institution are the very people who preach about such risks in the environmental context and warn that the actions of individuals affect the well-being of all. The natural world, they say, can stretch only so far before breaking as we tinker with the realities of its systems.

We understand little about how marriage has undergirded the order and prosperity we take for granted. We tamper with marriage at our peril.

Katherine Kersten is a Twin Cities writer and speaker. Reach her at kakersten@gmail.com.