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.