One theme in sociology is the idea that how a society is, just reflects the whims of the present leaders plus the cultural influences of past leaders. However, this is actually somewhat incorrect. And the evidence for this incorrectness is found in the existence of the social structure called 'family'.
Societal institutions are how essential societal tasks get carried out in an organized fashion.
Producing the next generation is an essential task. But dealing with sexual activity and plus the birth and care of children from sexual activity, are the essential societal tasks carried out by family or kinship institutions. Closely related is the institution of marriage. Here in the US and much of the world, we can come out and say that marriage is a socially legitimate sexual union with the price that the partners limit their sexual activity to only each other. Thus the offspring produced can be tied to that couple, and then that group of people are considered a family.