Supreme Court Justice Anthony Kennedy can't seem to understand why every political community on earth until the year 2000 recognized marriage as the union of a man and a woman. In his mind, this can be explained only by anti-gay "animus."

Justice Kennedy is wrong.

The state isn't in the marriage business because it cares about love or romance, but because the sexual union of a man and a woman can produce new life and this new life deserves a mother and a father. When a newborn isn't raised by the man and the woman -- the mother and the father -- who gave him or her life, social costs run high.

Today, let's look at what marriage is and why it matters. Tomorrow we'll consider how redefining marriage will undercut the public purpose and social function of marriage. On Thursday, we'll preview where the marriage movement goes from here.

Marriage exists to bring a man and a woman together as husband and wife to be father and mother to any children their union produces. Marriage is based on the anthropological truth that men and women are different and complementary, the biological fact that reproduction depends on a man and a woman, and the social reality that children need both a mother and a father.

There is no such thing as "parenting." There is mothering and there is fathering. Although men and women are each capable of providing their children with a good upbringing, typically there are differences in how mothers and fathers interact with their children and the functional roles that they play.

Dads play particularly important roles in the formation of both sons and daughters.

"The burden of social science evidence supports the idea that gender-differentiated parenting is important for human development and that the contribution of fathers to childrearing is unique and irreplaceable," Rutgers University sociologist David Popenoe explains.

"We should disavow the notion that 'mommies can make good daddies,' just as we should disavow the popular notion ... that 'daddies can make good mommies,' " Popenoe concludes. "The two sexes are different to the core, and each is necessary -- culturally and biologically -- for the optimal development of a human being."

Government recognizes marriage because it is an institution that benefits society in a way no other relationship does. Marriage is society's least restrictive means of ensuring the well-being of children. State recognition protects children by encouraging men and women to commit to each other and take responsibility for their children.

Social science confirms this. The best available research evidence shows that children fare best on virtually every examined indicator when reared by their wedded biological parents. Studies that control for other factors, including poverty and genetics, suggest that children reared in intact homes do best on educational achievement, emotional health, familial and sexual development, and delinquency and incarceration.

"It is not simply the presence of two parents ... but the presence of two biological parents that seems to support children's development," a study published by the left-leaning research institution Child Trends concluded:

"Research clearly demonstrates that family structure matters for children, and the family structure that helps children the most is a family headed by two biological parents in a low-conflict marriage. Children in single-parent families, children born to unmarried mothers, and children in stepfamilies or cohabiting relationships face higher risks of poor outcomes."

A study in the Journal of Marriage and Family concluded that "the advantage of marriage appears to exist primarily when the child is the biological offspring of both parents."

Promoting marriage as the union of a man and a woman doesn't ban any type of relationship: Adults are free to make choices, and they don't need government sanction or license to do so. All Americans have the freedom to live as they choose, but no one should redefine marriage for everyone else.

While respecting everyone's liberty, government rightly recognizes, protects and promotes marriage as the ideal institution for childbearing and childrearing.

-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."