The culture of enforced monogamy also involved closeted gay people getting married to people they had no sexual relationship with and hiding their real relationships, rampant alcoholism and domestic violence, many depressed and anxious people stuck in relationships they were unhappy in but felt social pressure not to leave, women with out-of-wedlock children and people who (for any number of reasons) never wanted to get married being ostracized by their communities, women systematically excluded from many careers and positions of social/political power, trusted institutions like the Catholic church covering up large-scale sexual abuse of children, etc. etc.

Cherrypicking a single summary statistic gives a far too rosy impression of past sexual/family culture. Ultimately if you go back far enough the original purpose of sexual mores related to marriage were primarily about clarifying and maintaining patriarchal land inheritance and designating women and children as chattel of the male head of household.