Meredith Fish, 35, and Adam Kriegstein, 41, of New York registered with Bloomingdale’s once they were engaged, largely because “some of my mom’s friends are uncomfortable giving money or not tech savvy to do a gift on the computer,” Ms. Fish said. But they also went with SimpleRegistry, where they asked for contributions toward a honeymoon adventure in Vietnam and Thailand. Guests could pony up for a day at an elephant camp, a romantic table set for dinner on a secluded beach or snorkeling with whale sharks.

Jason Dorsey, the chief strategy officer and a millennials researcher at the Center for Generational Kinetics in Austin, Tex., sees this as a change that is continuing to accelerate. With large student loans to pay, later marriages and often houses already purchased and filled with furnishings, millennials are bypassing traditional registries.

“Couples of this generation prefer experiences over stuff,” said Mr. Dorsey, who also works with retailers and companies that cater to millennials. “Less is more. This generation of couples live in smaller spaces and don’t need gifts. They would prefer a visit to a yoga retreat or tickets to a concert. They want more personal reflections of what they value.”

Dr. Arielle Kuperberg, an assistant professor of sociology at University of North Carolina at Greensboro, is researching the rise of cohabitation and how marriage in general has changed since the 1940s. “When people have lived on their own for years, it is hard to register when they marry,” Dr. Kuperberg said. “This generation of couples also cohabitate in great numbers, entertain casually, marry later. We call this the ‘independent life stage’ in sociological terms. They don’t need anything more for the house.”

Instead, she said, “They ask for cash because they often have no savings and may want to buy a new home together or be prepared for an emergency situation.”