open marriage vs matrimony

open marriage and matrimony both are nouns.

Word	Noun	Adjective	Verb	Adverb
open marriage	Yes	No	No	No
matrimony	Yes	No	No	No

As nouns, matrimony is a hypernym of open marriage; that is, matrimony is a word with a broader meaning than open marriage:

open marriage: a marriage in which each partner is free to enter into extraneous sexual relationships without guilt or jealousy from the other
matrimony: the state of being a married couple voluntarily joined for life (or until divorce)

Other hypernyms of open marriage include marriage, spousal relationship, union, wedlock.

open marriage (noun)	matrimony (noun)
a marriage in which each partner is free to enter into extraneous sexual relationships without guilt or jealousy from the other	the ceremony or sacrament of marriage
	the state of being a married couple voluntarily joined for life (or until divorce)