Prof. Teresa Collett in the Star-Tribune:
And while it is true that the most basic physical needs of the child can be met by almost any adult, common sense and social science tell us that, most of the time, the best people to care for the child are her mother and father.
Children in the United States who are raised outside of the home of their married, biological parents are two to three times more likely to suffer from psychological problems, such as depression, and engage in socially harmful behavior like delinquency and dropping out of high school.
Sociologist Paul Amato estimates that if the United States enjoyed the same level of family stability today as it did in 1960, the nation would have 750,000 fewer children repeating grades, 1.2 million fewer school suspensions, approximately 500,000 fewer acts of teenage delinquency, about 600,000 fewer kids receiving therapy, and approximately 70,000 fewer suicide attempts every year.
The legal institution of marriage exists primarily to encourage lifelong monogamous sexual unions of men and women, where we expect that children will result as often as not.