From
The Woman's Book 1894:
"Both men and women earn, as a rule, just what they are worth. The law of supply and demand comes into play as relentlessly here as elsewhere. If a woman wants more money than a man, she has only to do better and more work in the long run than the man, and she will get more pay as surely as business rules remain in force in the business world."
Funny, that. You know, right now, more than a century later, Danish labour union HK is pushing for equal pay in a very visible way
(article in English) .
The Woman's Book mentioned reasons for pay inequality such as: "women as a class of workers are beginners" and " [they] lack a heredity instinct for such work"
Perhaps we are just still inexperienced, compounded by the fact that our lack of a penis make us unable to pass on a hereditary instinct for men's work?