The sustainable city of the future will rest on the revival of traditional institutions that have faded in many of today’s cities. Ellen Moncure and Joe Wong first met in school and then fell in love while living in the same dorm at the College of William and Mary. After graduation, they got married and, in 1999, moved to Washington, D.C., where they worked amid a large community of single and childless people. Like many in their late 20s, the couple began to seek something other than exciting careers and late-night outings with friends. “D.C. was terrific,” Moncure recalled over...