LIKE many considerate boyfriends, Ed Vessel, a cognitive neuroscientist who lives in Brooklyn, bought a toothbrush for his steady girlfriend Diana Adams to keep at his apartment when she sleeps over. While Ms. Adams, a Cornell-educated lawyer, considered this touching, she was less pleased when she noticed the toothbrush that Mr. Vessel had bought for his other steady girlfriend when she slept over.

That Mr. Vessel had a second girlfriend was not the issue. All parties here are committed to polyamory, which for them means maintaining multiple steady relationships, with the knowledge and consent of all involved. The problem was that the other woman’s toothbrush was “a really fancy one that says ‘Primo’ on it, and mine is a junky one that says ‘Duane Reade,’ ” said Ms Adams, 29. For about a month, she was a little miffed every single time she brushed her teeth.

The two eventually talked  polyamory involves a lot of talking  and they now laugh about it. “I just decided that this was an example of a jealousy that is not warranted,” Ms. Adams said.

Polyamory gained a degree of cultural vogue in the sexual revolution of the 1970s, when books like “Open Marriage” made best-seller lists and swingers capitalized on the concept to justify experimentation. But while it failed to survive the era of fern bars for the mainstream population, a small but vocal collection of adherents  many borrowing the language of inclusion used in the gay rights movement  argues that polyamory can be a workable, responsible way to live.