Thus the heritage which group marriage has bequeathed to civilization is double-edged, just as everything civilization brings forth is double-edged, double-tongued, divided against itself, contradictory: here monogamy, there hetaerism with its most extreme form, prostitution. For hetaerism is as much a social institution as any other; it continues the old sexual freedom – to the advantage of the men. Actually, not merely tolerated but gaily practised by the ruling classes particularly, it is condemned in words. But in reality this condemnation never falls on the men concerned, but only on the women; they are despised and outcast in order that the unconditional supremacy of men over the female sex may be once more proclaimed as a fundamental law of society.

But a second contradiction thus develops within monogamous marriage itself. At the side of the husband who embellishes his existence with hetaerism stands the neglected wife. And one cannot have one side of this contradiction without the other, any more than a man has a whole apple in his hand after eating half. But that seems to have been the husbands’ notion, until their wives taught them better. With monogamous marriage, two constant social types, unknown hitherto, make their appearance on the scene – the wife’s attendant lover and the cuckold husband. The husbands had won the victory over the wives, but the vanquished magnanimously provided the crown. Together with monogamous marriage and hetaerism, adultery became an unavoidable social institution – denounced, severely penalized, but impossible to suppress. At best, the certain paternity of the children rested on moral conviction as before... Such is the final result of three thousand years of monogamous marriage.

— The Origin Of The Family, Private Property, and The State by Friedrich Engels (Page 75)