… unless one happens to be a Biblical Patriarch, in which case marriage equals one man and several women, or even one man, several women and free use of slave girls called concubines.
Or if one happens to be a legendary Israelite King then marriage equals one man and hundreds of women plus slave girls.
Or if one happens to be a Jew up until the tenth century, marriage could mean one man and several wives.
Much the same held in China for century upon century.
Or perhaps one happens to be one of the Toda, then marriage equals several men plus one woman. Or perhaps one is a nomadic Tibetan, then marriage equals two brothers and one woman.
Or maybe one is Kaingang, in which case one has options– one man plus one woman, one man plus several women, several women plus one man, or several men plus several women.
Or maybe one is Nayar, in which case marriage isn’t much about adding at all.
Or in large swathes of the pre-columbian New World, one could opt to marry a berdache.
A soldier in any number of cultures– among them the ancient Greek and the Japanese– could take a younger soldier for a ‘military wife’. The practise still continues in some African cultures, the !Kang, for example.
Really, these “Marriage = One Man + One Woman” people need a lesson in cultural anthropology.