David Blankenhorn, president of the marriage advocacy group the Institute for American Values, said married couples had become a minority largely because of the growing number of households made up of people who planned to marry or who used to be married.

Steve Watters, the director of young adults for Focus on the Family, a conservative Christian group, said that the trend of fewer married couples was more a reflection of delaying marriage than rejection of it.

“It does show that a lot of people are experimenting with alternatives before they get there,” Mr. Watters said. “The biggest concern is that those who still aspire to marriage are going to find fewer models. They’re also finding they’ve gotten so good at being single it’s hard to be at one with another person.”

But Pamela J. Smock, a researcher at the University of Michigan Population Studies Center, said her research — unaffiliated with the Census Bureau — found that the desire for strong family bonds, and especially marriage, was constant.

“Even cohabiting young adults tell us that they are doing so because it would be unwise to marry without first living together in a society marked by high levels of divorce,” Ms. Smock said.

A number of couples interviewed agreed that cohabiting was akin to taking a test drive and, given the scarcity of affordable apartments and homes, also a matter of convenience. Some said that pregnancy was the only thing that would prompt them to make a legal commitment soon. Others said they never intended to marry. A few of those couples said they were inspired by solidarity with gay and lesbian couples who cannot legally marry in most states.

Jennifer Lynch, a 28-year-old stage manager in New York, said she had lived on the Lower East Side with her boyfriend, who is 37 and divorced, for most of the five years they have been a couple.