Multilateral sexuality is an umbrella term that encompasses all of the sexual activities that individual members of a couple may, with the knowledge and consent of their partner, engage in with partners outside of their relationship. Activities such as swinging, partner swapping, and group sex are just a few examples of multilateral sexuality.
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The term “ethical non-monogamy” can be used in the same way as multilateral sexuality. It may be the preferred term because it explicitly explains that the idea of the non-monogamy in question is ethical (consensual). Secret infidelity or affairs carried on without the knowledge of one’s partner do not fall under the umbrella of multilateral sexuality