There are three reasons to be concerned about this dramatic shift in family life.

First, marriage is a commitment that cohabitation is not. Taking a vow before friends and family to support another person “until death do us part” signals a mutual sense of shared responsibility that cannot be lightly dismissed. Cohabitation is more fragile — cohabiting parents split up before their fifth anniversary at about twice the rate of married parents.  Often, this is because the father moves on, leaving the mother not just with less support but with fewer marriage prospects. For her, marriage requires finding a partner willing to take responsibility for someone else’s kids.

Second, a wealth of research strongly suggests that marriage is good for children. Those who live with their biological parents do better in school and are less likely to get pregnant or arrested. They have lower rates of suicide, achieve higher levels of education and earn more as adults. Meanwhile, children who spend time in single-parent families are more likely to misbehave, get sick, drop out of high school and be unemployed…

Third, marriage brings economic benefits. It usually means two breadwinners, or one breadwinner and a full-time, stay-at-home parent with no significant child-care expenses. Unlike Murphy Brown — who always had the able Eldin by her side — most women do not have the flexibility afforded a presumably highly paid broadcast journalist. And it’s not just a cliche that two can live more cheaply than one; a single set of bills for rent, utilities and other household expenses makes a difference. Though not necessarily better off than a cohabiting couple, a married family is much better off than its single-parent counterpart.