non-monogamy
The researchers concluded that people in nonmonogamous marriages are no happier or no more miserable than those in monogamous marriages.
Mule deer are non-monogamous.
I was in a non-monogamous relationship for three years.

SMART Vocabulary: related words and phrases

Marriage, cohabitation & other relationships
adulteress
affinal
as husband and wifeidiom
be an itemidiom
be shacked upidiom
have an affair
have something going with someoneidiom
hear wedding bellsidiom
hitched
honest
matrimonially
matrimony
monogamist
monogamous
ménage à trois
shack
shack up
situationship
something
together