So who are the mysterious people these nonmonogamous couples are sleeping with?

What would it mean to be in someone else's open relationship as a single woman?

"It was very clear what the hierarchy was, but he called us both his 'girlfriends,'" she says.

Is it possible to be happy as a "secondary," as wince-inducing as the word is?

Beth*, a 37-year-old therapist in San Francisco who's currently dating a couple (sexual with the man, "romantic" but not sexual with the woman), is of two minds about the settling question.

When Ivy*, a 35-year-old activist, lived in New York, her relationships never seemed to work out.

She dated the way a lot of people date in the city, juggling multiple partners without any real forward movement.

She became his polyamory protégé, and has since had four open relationships.

In her second open relationship, her boyfriend already had a serious girlfriend.

In the open-relationship world, there's a term for this: "couple privilege." It was introduced to the lexicon by Franklin Veaux, coauthor, with Eve Rickert, of 2014's .

They define it as "external social structures or internal assumptions that consciously or unconsciously place a couple at the center of a relationship hierarchy or grant special advantages to a couple." You can imagine how this plays out in practical terms.

Would it always seem like the dreaded settling, a lesser version of what one should truly want?

Does it always mean wasting a limited amount of emotional and psychological bandwidth?

The focus is always on the couple—how their adventures in nonmonogamy fuel their partnership and heighten their sex lives; how they're able to navigate sleeping with others without breaking their sacred union.