In our part of the world, where monogamy is in force, to marry means to halve one’s rights and to double one’s duties. When the laws granted woman the same rights as man, they should also have given her a masculine power of reason. On the contrary, just as the privileges and honours which the laws decree to women surpass what Nature has meted out to them, so is there a proportional decrease in the number of women who really share these privileges; therefore the remainder are deprived of their natural rights in so far as the others have been given more than Nature accords.

For the unnatural position of privilege which the institution of monogamy, and the laws of marriage which accompany it, assign to the woman, whereby she is regarded throughout as a full equivalent of the man, which she is not by any means, cause intelligent and prudent men to reflect a great deal before they make so great a sacrifice and consent to so unfair an arrangement. Therefore, whilst among polygamous nations every woman finds maintenance, where monogamy exists the number of married women is limited, and a countless number of women who are without support remain over; those in the upper classes vegetate as useless old maids, those in the lower are reduced to very hard work of a distasteful nature, or become prostitutes, and lead a life which is as joyless as it is void of honour. But under such circumstances they become a necessity to the masculine sex; so that their position is openly recognised as a special means for protecting from seduction those other women favoured by fate either to have found husbands, or who hope to find them. In London alone there are 80,000 prostitutes. Then what are these women who have come too quickly to this most terrible end but human sacrifices on the altar of monogamy? The women here referred to and who are placed in this wretched position are the inevitable counterbalance to the European lady, with her pretensions and arrogance. Hence polygamy is a real benefit to the female sex, taking it as a whole. And, on the other hand, there is no reason why a man whose wife suffers from chronic illness, or remains barren, or has gradually become too old for him, should not take a second.

He's saying that giving women equal rights would result in less marriage in the 1800's when feminism was but a small movement. Over a hundred years later his prophecy has come true. Food for thought. Great man, check out his books.

The World as Will and Representation Vol.1 and there's a Vol.2 also.