simonedebeauvoirI am so beyond tired of English speakers misinterpreting this quote and making it mean the exact opposite of what Beauvoir meant by it.
Here is the explanation that immediately follows it in The Second Sex:

  “One is not born, but rather becomes, a woman. No biological, psychological, or economic fate determines the figure that the human female presents in society: it is civilization as a whole that produces this creature, intermediate between male and eunuch, which is described as feminine.”

I don’t know how much clearer she can be: she defines “woman” as “the figure that the human female presents in society” and goes on to criticise the fact that society has made this figure into something “intermediate between male and eunuch” and called it feminine. To Beauvoir, “woman” is a harmful social construct forced on females, so this word doubly doesn’t apply to transwomen, who a) are not female and b) are not forced into womanhood.

When people use “one is not born, but becomes a woman” to mean “anyone can become a woman”, they
– are conveniently forgetting the part where the political construct of “woman” is inextricably linked to the condition of being born a human female (I mean, the first chapter of that book is called The Data of Biology…) and the part where Beauvoir is criticising this man-made concept of “woman” as well as the, in her words, “hierarchy of sexes” (also known as ~spectrum of genders~) it helps create; and
– are essentially saying that transwomen are feminine eunuchs. That is what “woman” is in this book, a restrictive patriarchal construct that could best be described as an inferior, castrated male with feminine clothing and behaviour, and that female humans are not born as (as patriarchy would like us to believe) but are forced to grow into, which is the main component of their oppression.
(Please keep in mind that this was written in the 1940s when the belief that women were naturally inferior and born to serve men wasn’t publicly challenged at all. When everyone defines “woman” as “naturally inferior”, saying “the human female is not born a woman, she is made one” is revolutionary.)

The point of “One is not born, but rather becomes, a woman” was not that male humans can also become women despite being born male, it was that female humans should not be forced to become a “woman” as defined by our patriarchal world. But trust our modern-day “feminists” to take a quote about female oppression and make it about male fantasies instead.

It is blatant gender criticism by SdB so you can see why it would need to be repurposed by men to serve men.

-Found on Ses Purs Ongles.