That was the original notion behind giving different tax treatment to married couples than to singles as well as having employers include spouses and children in group insurance coverages.
In this respect, polgamy and polyandry are, unlike same sex unions, at least still founded on begetting and raising children. Many cultures in many times and places have or do practice polygamy. I know of none that did so before the last few decades anywhere. Oh there was homosexuality and even legally and socially condoned and/or incouraged homosexuality, but AFAIK, not homosexual marriage.