To most people, the idea of their loved one dating and having passionate sex with other people is repellent. But for two married couples -- with children -- in Somerville, Mass., and thousands more across the country, this is the happy, stable norm.
Sierra Black, 33, is a writer for a popular parenting website. She's been married to Martin, 47, for almost nine years. Martin is a research scientist at a nearby university. (Martin and others in this article will be identified only by first...Full Story