What did Simone de Beauvoir mean by labeling women as "the second sex"?
When Simone de Beauvoir describes women as the "second sex," she means that maleness is assumed, that men are the standard from which women are the deviation. A man does not have to proclaim that he is a man as a part of his identity, but a woman does. She argues,
If I want to define myself, I first have to say, "I am a woman"; all other assertions will arise from this basic truth. A man never begins by positing himself as an individual of a certain sex: that he is a man is obvious.
Then, people will make assumptions based on the woman's sex. For example, someone might claim that she believes something because she is a woman, but a person would never say that someone believes something because he is a man. To be a man is to have one's sex be invisible, but to be a woman is to have one's sex take front and center stage of one's identity. Beauvoir continues,
The relation of the two sexes is not that of two electrical poles: the man represents both the positive and the neuter to such an extent that in French hommes designates human beings [. . .]. Woman is the negative, to such a point that any determination is imputed to her as a limitation, without reciprocity.
So, it is not that man is positive and woman is negative, but man is also neutral, the default. Consider how often we use the pronoun "he" when we discuss a generic person of unknown sex, or words like mailman, fireman, or chairman. We find ourselves having to retrain our brains to say mail carrier, fire fighter, or chairperson. Our brains default to the assumption of maleness, and we only move to femaleness in a secondary step.
Not equal, as their sex not equal seemed;
For contemplation he and valor formed,
For softness she and sweet attractive Grace,
He for God only, she for God in him:
Milton's description of Adam and Eve in Paradise Lost delineates precisely the distinction Simone de Beauvoir makes in The Second Sex. Man comes first, as the Bible says he was created first. He is closer to God and more godlike in his attributes. Woman is an afterthought, continually defined in relation to man.
Man is also the "default" sex, the one that includes the other. Mankind includes womankind. This is even more striking in French, since the plural word "they" can be either "ils" or "elles." If it is a mixture of men and women, the word is "ils," the masculine form.
de Beauvoir argues that women are always the second sex both in our consideration and our estimation. We think of women second, and we think them less valuable. If we hear that Dr. X has made a discovery, or that this book was written by X, we immediately assume the person to be a man. Consider all the female writers who have used male pseudonyms. Also, a woman's emotions are often attributed to her sex rather than to the issue at hand: an angry man is assumed to have a reason for his anger, while an angry woman is merely being hysterical (the word "hysteria" comes from the Greek for "uterus" and has historically been regarded as a disorder specific to women).
By the "second sex," Simone de Beauvoir means that women are conditioned by society to understand themselves only in relation to men, whereas men are defined on their own terms. In other words, a woman's identity is defined by her relationship to a man, i.e. her husband. She traces the intellectual roots of this development through the centuries, arguing that science, religion, legal systems, and other institutions have been culpable in making women define themselves in this way. More importantly, though, she discusses how the stages that society has attached to the maturation process of women have also contributed. In short, women are made to feel like subjects or "others," a condition made even worse by marriage.