I;m afraid you're conflating two-parent family with monogamy. Monogamy is certainly not a modern invention, two-parent family is, especially the "breadwinner-homemaker" model, that was a social experiment, you got all these couples shipped to distant suburbs among strangers. Traditionally there was multi-generational community to look after the kids, you know, "it takes a village". Neither was Eve, Sarah or Rachel a stay at home mom, they had neighbors and extended families, today it's all the burden is on the mom alone, they rarely have the husband's support.