The title alone is outdated. We have the third-sex now.
Actually we have an endless number of genders now, which is a great thing!
That doesn't make The Second Sex irrelevant or outdated.
The title comes from the idea that a category of identity has to be opposed to another category in order to exist. The more powerful group often defines the differences between the categories, makes them more extreme and guards them in order to keep the balance of power the way it is.
Simone de Beauvoir reveals the social mechanisms in which men have made women the 'other sex'. She writes about the history of patriarchal structure, and explains how the biological differences between men and women were used as a justification for a systematic discrimination of women throwout the history. Later she argues that the categories of 'men' and 'women' are cultural, rather than natural. She gives examples of how girls and boys are being directed and designated from a very young age to fulfill a traditional gender role.
Also, she makes a link between sexism and the suppression of other minority groups. Discrimination in the name of false biological/natural justifications is very common. It stands in the basis of racism, homophobia, etc. That alone makes the book super relevant for the third sex, don't you think?
This book is brilliant. I think everyone should read it, especially people who are interested in feminism, gender, sexuality. The ideas and examples she's basing her theories on come from a wide range of sources, and from many different disciplines. She defines in that book for the first time many terms and ideas that are now the foundation stones of feminism. For me, reading it helps me make sense in the world I see around me, though sometimes it makes me feel so helpless too. It's true though that the situation today is different than then, in a good way. Some of the examples she gives are less relevant nowadays, and I notices that it's usually so when she talks about women. I guess women have changed more than men, and I hope men are going to catch up soon. Either way- the general ideas, the mechanisms she describes, the problems and the possible solutions to them are very relevant still.
I could go on and on and on, but I will stop bore you now