Katherine Woodward Thomas, the Berkeley-based family therapist who coined the term “conscious uncoupling” more than a decade ago, said that the key to making the next stage work will be honesty, as well as clear communication about the situation with their two children, who are in their 20s. That will be true, she and other experts said, no matter how this turns out — whether one person falls in love and ultimately wants a divorce, or if they find a way to have a more open relationship.

The first step down a long road of untangling the emotional bonds of marriage will be to build a fuller identity outside of politics and the home, Ms. Thomas said. And that may well be harder for Mr. de Blasio than Ms. McCray, who had a proud personal identity as an out lesbian before marrying her spouse.

“He needs to go out now and to develop himself — and not just on the dating scene,” Ms. Thomas said. “He’s got a big makeover ahead of him.”

The biggest cultural contribution of their separation story, Mr. Savage said, would be if it sparks a conversation for other couples who, after decades together, seek respectful ways to value their marriages while opening the door to something new.

A few blocks from Mr. de Blasio’s YMCA, neighborhood residents generally had good things to say about his and Ms. McCray’s announcement. Some commented on how “Brooklyn” it was and thought it could set an example for other couples in similar situations.

“It’s inspiring what they’re doing,” said Kent McVey, 65, while on a walk in Park Slope with his wife of 43 years, Laura, 67, and their two dogs. “To me, that just shows that there is a deep, deep level of respect and friendship. If they don’t stay together and they end up dating other people and they marry someone else, they’re going to stay friends forever. What other way would you want it? I think that’s incredible. I love it.”

Olivia Bensimon contributed reporting.