Explicit symmetrical polyamory has never emerged stably in history so far. It's worth asking why. Maybe this is coincidence; maybe something has changed now that will be more conducive, but I think it's worth pointing out.

Primates (including humans) raised in stable, supportive environments are more friendly, trusting, willing to take risks. Those who grew up desperately alone, or with only a few allies-of-convenience who might run off as soon as costs outweighed benefits or better prospects appeared elsewhere, are less friendly, trusting, and willing to take risks. This mechanism evolved because using either strategy in the opposite environment means being isolated from the support of your peers and/or murdered at a young age, which is strongly selected against. Polyamory requires a large population of friendly, trusting-and-trustworthy potential partners; modern economic and political developments have produced an environment (in some parts of the world, anyway) sufficiently stable and prosperous that such a population can emerge and thrive.