Polyfidelity

The term polyfidelity was popularised in the 1970s by the Kerista commune in San Francisco, the same community credited with coining compersion. Kerista's structure required that all sexual relationships happen inside the group; that closed-circle principle survived the commune's dissolution and was carried into modern polyamory vocabulary. Wikipedia's polyfidelity article traces earlier antecedents back to the 19th-century Oneida community, which practised complex marriage across the entire group rather than within negotiated subsets.

In contemporary practice, polyfidelitous structures most often appear as triads (three partners) and quads (typically two couples who have integrated into a single closed unit), though larger configurations exist. Members commit to sexual exclusivity within the group in the same way a monogamous couple does, which has two practical consequences. First, the negotiation of new partners — a routine question in open polyamory — is removed from the table. Second, STI risk is bounded by the group, which in turn is what makes fluid bonding across all members medically credible.

The arrangement is widely described as offering the emotional stability of monogamy with the relational density of polyamory, but it is also widely described as fragile to changes in any single member's wants. If one partner develops interest outside the closed group, the whole structure has to renegotiate from scratch — there is no half-step between polyfidelity and open polyamory. Communities such as More Than Two publish extensively on the failure modes specific to closed multi-partner structures, particularly the asymmetry that emerges when one couple in a quad is more invested than the other.