The author asks on what basis the constitution is supported when it states that men and women should be treated unequally. She argues that the rights of both sides deserve the same treatment and reasoning, even if men say they are doing what is best for women in an attempt to justify the oppression.
The quotation in the question above was taken from A Vindication of the Rights of Woman, written by Mary Wollstonecraft (mother of Frankenstein’s author Mary Shelley) in the 18th century. She is considered to be one of the founders of feminism.