And now that large-scale industry has taken the wife out of the home onto the labour market and into the factory, and made her often the breadwinner of the family, no basis for any kind of male supremacy is left in the proletarian household, except, perhaps, for something of the brutality toward women that has spread since the introduction of monogamy. The proletarian family is therefore no longer monogamous in the strict sense, even where there is passionate love and firmest loyalty on both sides and maybe all the blessings of religious and civil authority. Here, therefore, the eternal attendants of monogamy, hetaerism and adultery, play only an almost vanishing part. The wife has in fact regained the right to dissolve the marriage, and if two people cannot get on with one another, they prefer to separate. In short, proletarian marriage is monogamous in the etymological sense of the word, but not at all in its historical sense.

— The Origin Of The Family, Private Property, and The State by Friedrich Engels (Page 78)