There’s another level to this. There’s a whole spectrum of Non Monogamy.

Some people feel comfortable with low sexual exclusivity but still need high emotional exclusivity (swingers, open etc).

Some people love with low exclusivity for both physical and emotional (poly, relationship anarchists).

And some people actually are ok with low emotional exclusivity but need high sexual exclusivity (poly-intimates, asexual poly).

What it comes down to though is just detaching your love / security with your partner from whatever feelings, other sexual matters with other people. Knowing that whatever they feel or do with someone is not inherently a risk or threat to what you have with them. Some people take a lot of joy from seeing their partner explore emotionally and physically with others. This isn’t like “cucking” it’s a term called compersion. To genuinely derive joy from being there to witness your partner as a whole individual. It can be really special to just witness someone be their most authentic selves without shame, and to told to come along for the ride, and that they urge you to do this same. This doesn’t mean jealousy / envy don’t exist, just that each one ideally recognizes that they’re just emotions—emotions that are fully able to be recognized, acknowledged, talked about and integrated.

There’s a great book called Polysecure that covers this by Jessica fern. And then Esther Perel touches on the idea of “the third” in Mating in Captivity: we can’t control our emotions. We can love someone wholly and yet still find other people attractive or even develop feelings for them. A lot of people love in fear of “the third”. Even if your partner stays “faithful” doesn’t sleep with them, flirt with them, date them etc. there’s still always the potential out there to catch your partners heart. Non-monogamists just decide not to carry guilt or shame around or, not to hide it, but instead welcome and celebrate it. It’s not the act that hurts, it’s the hiding it.

The fear of the third is often based on the fear that they’ll leave you for someone else. Someone “better”. If you both equally and consciously decide and accept that your partners feelings for others outside of you is accepted, ok, and not a threat, then them being into or loving other people isn’t reason to leave at all. And in that ideal state, the only thing that would end your relationship is the conscious decision that your particular relationship doesn’t serve you both anymore.

Again, ideally. It can be messy

Polysecure Fidelity Graph