The definition of marriage varies widely across cultures.
If you're old-school-Mormon, it's multiple women, one man. If you're southern African before the Christian massacres, it's also one man multiple women, except sometimes it was multiple women and no men at all. One of the women would be the "husband." While in South American cultures there were men who would take on feminine roles. Very ancient Greeks had multiple men in committed relationships, although the latter part of that period they would take wives and then just sleep around with men. Meantime I know several people in open marriages, some of which are closed circle -- effectively polygamous marriages in small mixed gender groups.
The revisionist idea that marriage is normally between one man and one woman is naive at best, outright dishonest at worst. People are complicated, and so are their relationships.