Marriage? Let me think about it
And what about the security of having a lifelong companion as you age? DePaulo, who is 57 and happily never married, said studies show married couples spend less time than singles calling, writing and visiting with their friends, neighbors and extended family. Putting all their eggs in the marital basket can make married people more vulnerable if something happens to their spouse, she said.

Pamela Haag, author of the new book "Marriage Confidential" (Harper Collins), believes marriage isn't dying so much as adapting to a "post-romantic" spirit that doesn't assume a spouse must be your end-all, be-all forever, which has become a tall order now that people are living so long.

Instead of taking or leaving the institution as is, some couples are adapting marriage rules to fit what they need, be it by loosening expectations of monogamy or committing just to co-parent until the kids are grown up, Haag said.

Rather than fret about the potential pitfalls of shifting marriage expectations, Gerson said it would be more helpful to create a culture that addresses the new challenges, such as more flexible jobs and social policies that treat unmarried and married people equally.

"For better or worse," Gerson said, "these changes are here to stay."

What about the kids?

At the National Marriage Project, a research initiative at the University of Virginia, a primary concern is how postponing or opting out of marriage affects kids, 24 percent of whom are now born to unmarried cohabiting partners.

In a just-released report, the group called cohabiting couples "the largest unrecognized threat to the quality and stability of children's family lives." Cohabiting couples with kids are more than twice as likely as married couples to break up before the child is 12, the report said, and family instability has been linked to school failure, behavioral problems, drug use, physical abuse and loneliness.

W. Bradford Wilcox, director of the project, worries that when young adults hold out for a "soul mate" marriage, rather than viewing marriage as a social good, childbearing becomes a separate, spouse-less journey. In a 2010 Pew Research survey, 52 percent of 18- to 29-year-olds said being a good parent is "one of the most important things" in life, while just 30 percent said the same about having a successful marriage, suggesting they're increasingly disconnecting the two institutions. (When young adults were asked the same questions in 1997, 42 percent said parenting and 35 percent said successful marriage.)

While the vast majority of kids in any family arrangement do just fine, on average children thrive better in an intact married household, Wilcox said.

But even putting kids aside, Wilcox challenges attitudes that marriage is merely a piece of paper. The collective ritual sends an important signal to couples and their community that they take certain norms seriously, he said, and the outside support and accountability helps the health of relationships.

"You'd be a lot more concerned about seeing a married friend who is two-timing than a cohabiting friend who is cheating on his girlfriend," Wilcox said.

Other scholars dispute the prevailing consensus that marriage is better for kids and couples than other arrangements.

Judith Stacey, whose book "Unhitched" explores marriage and family setups in different cultures, said kids need a consistent, attached, decent relationship with at least one adult, preferably two, but marital status doesn't matter. What's more damaging to kids not living with their married biological mom and dad is living in a culture that stigmatizes it, she said.

Why the decline?

The brunt of the marriage decline has occurred among moderately and least-educated Americans, perhaps because the loss of manufacturing jobs has left people feeling financially ill-equipped to marry, but also because shifting social mores have left them less marriage-minded, said W. Bradford Wilcox, director of the National Marriage Project. College-educated Americans have the most stable marriage culture.

Between 1960 and 2008, marriage rates dropped to:

64% from 76% among college-educated Americans.

50% from 72% among Americans with some college.

48% from 69% among Americans with a high school education or less.

Source: Pew Research Center

— A.E.R.