Miles Bratton, 47, said she would consider forming a domestic partnership with Anne-Marie Taylor, 43, whom she called her “platonic lifemate.”

The status would allow them to buy a house together and share benefits, like health insurance, but also to have outside romantic partners, or add a third “nesting partner” if they wished. Ms. Taylor said they had long held back from registering as domestic partners because the language her workplace used seemed to require that they be romantic partners.

“That has not felt right, so we haven’t done it,” she said. “Somerville is coming out and saying, ‘Hey, family can be a lot of other things, other than just two people.’”