Why is the nearlywed trend so big right now? Some women say it's because they simply don't consider the marriage certificate essential anymore. (In a May 2012 Glamour survey of 2,100 women, nearly half called marriage "outdated.") Sophia Lormeus, 31, feels that way. She and her partner of four years are expecting a baby, but she has no plans to walk down the aisle—and resents those who imply she should. "Society has made being married more important than having a great relationship, and I already have that," says the human resources consultant in Montreal. "He treats me so well and will be such a fantastic father, it feels insulting to think we are not truly committed just because we're not married."

For others, nearlywedding is less scary—and permanent—than marriage. "We are in an age of uncertainty," says Jennifer M. Silva, Ph.D., author of a new book on entering adulthood, Coming Up Short. With education costs soaring, jobs disappearing, and divorce all around them, many young people are afraid to depend on others. "That seeps into relationships too," she says, "so any real commitment becomes yet another risky venture." Which is why nearlywedding is a perfect middle ground. "It used to be that you married young, lived in a crappy apartment, finished school, and grew into adulthood together," says Karen Benjamin Guzzo, Ph.D., an assistant professor of sociology at Bowling Green State University in Ohio, who specializes in nontraditional families. "Today people feel a real need to get their career in a good place and be financially secure first, which takes a long time in our economy. In the meantime, people still fall in love and want to be with somebody."

No wonder, then, that the nearlyweds Glamour spoke to think playing husband and wife is the perfect test run: "You can't hide your idiosyncrasies when you live together, so it confirms either that you'll make good partners or that you're not really compatible—before you jump in," says Kaitlin Dorn Severini, 29, a production editor in New York City. "It can keep you from making a huge mistake."

For worse: The "easy out" relationship

So what's the downside? Some experts question the nearlywed claim that everything-but-marriage is as good as marriage. "Research shows that one of the top predictors of a successful relationship is having a high level of commitment, and that high level is rare without signing a contract," says Robert Epstein, Ph.D., a senior research psychologist at the American Institute for Behavioral Research and Technology in Vista, California. "When something goes wrong as a nearlywed, it can make you doubt the relationship. But when you're married, you think, OK, he's being a jerk, but we'll figure this out. Having that piece of paper keeps you working hard to make the arrangement a success."

Dr. Saltz is more measured, but she too suspects that it's rare for a nearlywed couple to be as committed as a wedded one. "Some people have a kind of moral contract where they'll stay in the relationship, married or not," she acknowledges. "Others, however, deep down, do want an escape hatch, and this seems like a way to have your cake and eat it too. But the truth is, anytime you have financial troubles, anytime someone passes by who looks a bit better than your other half, there's that thought: I can get out."

"People nearlywed for all kinds of reasons, and his may not be yours."

—relationship expert Andrea Syrtash

Danielle Marshall, 26, says her nearlywed state felt like limbo to her—and that wasn't a good thing. "When my boyfriend and I moved in together, we talked about eventually marrying and having kids," says the information-technology specialist in Mesa, Arizona. But "eventually" turned into five years, a dog, two cars, savings for a down payment on a house— and still no wedding ring. "Every time I brought up marriage, he'd blow it off, like, Why do we need a piece of paper? He told me we'd be together forever, but I knew he still felt like he had the option to leave."