The books are popping up everywhere: "Unmarried to Each Other," "The Living Together Kit," and the "Complete Idiot's Guide to Living Together."
So are the employee benefits. More than a third of the Fortune 500 companies now offer them to their workers' domestic partners. And goods and services - from life insurance to rental cars - are targeted at unmarried couples.
The reason? Corporate America has figured out that the unconventional branches of the nation's family tree are growing more prominent and more diverse - even as the United States appears to be taking a slightly more conservative stance politically.
For example: Not only did the number of people living together grow 72 percent during the 1990s, but 43 percent of them are also raising children, according to a census report released today. Even same-sex couples are moving into child-rearing mode, the report found. A third of female-partner households in 2000 included kids, as did a fifth of male ones.
"The message is that child-rearing outside marriage is not just something that poor people are doing," says Andrew Cherlin, a public-policy professor at Johns Hopkins University in Baltimore. "It's spreading to the middle class."
This growing diversity of family structure is heating up the debate over whether the nation - and children - benefit from such unconventional arrangements. Financially, the answer seems to be no. Socially, the evidence is sparse and hotly contested.
Consider the financial angle. The numbers indicate that marriages promote more stability than couples who live together.
The results look particularly striking for mothers. Typically, a mother moving into marriage saw her living standards soar 65 percent above those of single parents living with no other adult, says Robert Lerman of the Urban Institute in a study last year. The married mother also enjoyed a 50 percent boost over single parents living with another adult and 20 percent over cohabiting adults.
Even pregnant women entering so-called shotgun marriages did 38 percent better on average and saw 20 percent lower variability of living standards than women who did not marry before the birth of their first child.
"Women who began their motherhood in a married state do better economically than other groups of mothers," Mr. Lerman concludes. The same seems true of fathers. Using 1990 census data, a Purdue University researcher found that married men averaged 14.1 percent more in wages than unmarried men.
The social effects of parenting by unmarried couples seem more controversial. For example, many researchers have come to the conclusion that traditional marriages remain the best option for raising children, as long as they're good marriages. They last longer than other relationships. They're more economically stable.
"There is growing evidence that the ideal context to promote child well-being is a healthy, conflict-free marriage involving a couple living with their common, biological children," concluded the Center for Research on Child Wellbeing at Princeton University in a research brief last June.
But other groups claim the impact of parental arrangements - even on children of same-sex couples - is minimal. Last year, for example, the American Academy of Pediatrics came out in favor of gay adoption, concluding that the children born to or adopted by such couples deserved two legally recognized parents. The group pointed to "a considerable body of professional literature that suggests children with parents who are homosexual have the same advantages and the same expectations for health, adjustment and development as children whose parents are heterosexual."
That stand initiated a firestorm of protest by traditional family groups, who pointed to other studies that suggested problems of sexual identity and promiscuity.
In reality, there is not much information on the effects on children. The boom in lesbian couples with children took place some 10 to 15 years ago, says Beth Teper, executive director of Children of Lesbians and Gays Everywhere, a San Francisco-based advocacy group. "In the last five to 10 years, there's been more of a gay-dad boom," she says.
For all the brouhaha over gay couples, they represent some 594,000 households, less than 1 percent of the US total, according to the census report. (Gay advocates, however, argue that their numbers remain severely undercounted.) Unmarried couples represent a much larger group - 5.5 million households, up from 3.2 million in 1990. But that's still only about 5 percent of total households.
By contrast, married couples account for 52 percent of US households - down slightly from the 56 percent in 1990.
The typical married couple tends to be a little older (late 40s) than same-sex couples (early 40s) and much older than cohabiting couples (mid 30s). Part of the reason is that cohabiting relationships typically don't last very long, says Professor Cherlin of Johns Hopkins, either because they break up or lead to marriage. Thus, living together may be serving more as a prelude to marriage than a substitute for it, he adds.
Not surprisingly, married couples are less likely to live in central cities than in the suburbs or rural areas. They're more prevalent in the Midwest than the Northeast.
California, by contrast, contained 12 percent of unmarried-partner households and 16 percent of same-sex partner households.