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Non-monogamy (or nonmonogamy) is an umbrella term for every practice or philosophy of intimate relationship that does not strictly hew to the standards of monogamy, particularly that of having only one person with whom to exchange sex, love, and affection. Therefore, in that sense "nonmonogamy" may be as accurately applied to infidelity and extramarital sex as to group marriage or polyamory.
More specifically, "nonmonogamy" refers to forms of interpersonal relationship, intentionally undertaken, in which demands for exclusivity (of sexual interaction or emotional connection, for example) are attenuated or eliminated. Individuals may form multiple and simultaneous sexual or romantic bonds. This stands in contrast to monogamy, and yet may arise from the same psychology.
The term has been criticized as implying that monogamy is the norm, and thus other ways of relating are deviant and therefore somehow unhealthy or immoral.
Many terms for non-monogamous practices are flexible in definition, being based on criteria such as "relationship" or "love" that are themselves questionably defined. There are forms whose practitioners set themselves apart by qualifiers such as "ethically non-monogamous" with the intent of distancing from the deceit or subterfuge they perceive in certain other relational forms.
As well, usage creates distinctions beyond the raw definitions of the words. Thus, even though some relationships might technically be considered both polygamous and polyamorous, "polygamy" usually signifies a codified form of multiple marriage, based on established religious teachings, while "polyamory" is based on the preferences of the participants rather than social custom or established precedent.
Forms of non-monogamy are many, and can overlap, a few being:
- casual relationship — a physical and emotional relationship between two unmarried people who may have a sexual relationship
- cuckoldry — a person has sex with another individual without the consent of their partner(s) or purposefully excludes them from sex
- group marriage — several people form a single familial unit, with each considered to be married to one another (popularized to some extent by novelists Robert A. Heinlein and Robert Rimmer)
- group sex and orgies involving more than two participants at the same time
- line families — a form of group marriage intended to outlive its original members by ongoing addition of new spouses
- ménage à trois — a sexual (and sometimes domestic) arrangement involving three people
- open relationship (incl. open marriage) — one or both members of a committed (or married) couple have the express freedom to become sexually active with others
- polyamory — participants have multiple romantic partners
- poly families — similar to group marriage, but some members may not consider themselves married to all other members
- polyfidelity — participants have multiple partners but restrict sexual activity to within a certain group
- polygamy — one person in a relationship has married multiple partners
- polyandry — a woman has multiple husbands
- polygyny — a man has multiple wives
- plural marriage — a form of polygyny associated with the Latter Day Saint movement in the 19th-century and with present-day splinter groups from that faith. It is also associated with an evangelical splinter group which advocates Christian Plural Marriage