It used to be called “shacking up.” Now it’s just another lifestyle choice.

Or so it appears from federal data released on July 28. It shows a big jump in the number of unmarried opposite-sex couples living together -- from less than 1 million 30 years ago to 6.4 million in 2007, or almost 10 percent of all opposite-sex U.S. couples.

Does it matter? Not to the 47 percent of people in a USA Today/Gallup poll who said that cohabitation made “no difference” to the children of cohabiting couples. At a time when same-sex “marriage” has become a wedge issue in California and other states, this trend is troubling, to say the least.

Cohabiting couples may dismiss marriage as old-fashioned -- a “piece of paper” that pacifies parents but has no practical value. In fact, a growing body of social science research shows that the intact family -- defined by countless generations and myriad cultures to mean a man and a woman who marry, conceive and raise their children together -- best ensures the welfare of society in general and children in particular.

It says a lot about the decline of our culture today that such an observation even needs to be made. Once, the family’s central role in our society was a given. Past U.S. presidents, such as Theodore Roosevelt and Ronald Reagan, understood that families are, as Reagan put it, “the bedrock of our society.”

Indeed, Reagan underscored the value of the family when defending his tax policy in a 1985 address to the nation. “There is no instrument of hard work, savings, and job creation as effective as the family,” he said. “There is no cultural institution as ennobling as family life. And there is no superior, indeed no equal, means to rear the young, protect the weak, or attend the elderly. None.”

Again and again, the data bears this out. Adolescents in intact families, for example, perform better on a number of measures when compared with their counterparts in non-intact families. As family expert Jennifer Marshall of The Heritage Foundation notes, children in intact families “have better health, are less likely to be depressed, are less likely to repeat a grade in school, and have fewer developmental problems.” Their peers in non-traditional households, meanwhile, “are more likely to experience poverty, abuse, behavioral and emotional problems, lower academic achievement, and drug use.”

Small wonder that Hillary Clinton, in her book “It Takes a Village,” wrote that her “personal wish” was for “every child [to have] an intact, dependable family.”

This is not to say that being raised in an intact family is a guaranteed ticket to success, or that children in non-traditional families have no hope of doing well. But as a group, the odds strongly favor the children raised in intact families. It’s really a question of giving our children the best possible chance to succeed. Cohabiting couples may wish it were otherwise, but in general their lifestyle doesn’t serve their children -- or the wider society in which they are raised -- as well as the traditional family does.

What is it, exactly, that makes the intact family such a strong force for good? Among other things, it gives its members a sense of belonging -- a desire that appears to be imprinted on every human heart. An intact family, more so than other types of households, imparts that belonging and gives its members a firm foundation for acceptance and success.

When that foundation is damaged, the need for belonging doesn’t go away. As Heritage expert Ryan Messmore has noted, it often is simply transferred to the state, which is happy to assume (in a decidedly imperfect manner, to be sure) the functions of the intact family. Not surprisingly, liberal politicians have long tried to win votes by using language that appeals to the urge to belong. President Lyndon Johnson spoke of America as a “family,” as did Walter Mondale when he ran for president in 1984. Unfortunately for liberals, nothing -- not even the most well-intentioned nanny state -- can replace the traditional family.

The bottom line is pretty simple: Society needs a critical mass of intact families to function effectively. That’s why the debates about same-sex “marriage” and cohabiting couples merit more than a shrug of the shoulders. Don’t bite your tongue and hope these issues will go away. They matter deeply -- and they need people willing to speak up for traditional values. For the sake of our country, let’s hope more people summon the courage to do that.