Researchers were at pains to point out that it is not the first time that Frank wrote about sex in her diary.
It turns out the pages contained four jokes about sex that Anne herself described as "dirty" and an explanation of women's sexual development, sex, contraception, and prostitution.
Frank's candid words on sex didn't make it into the first published diary, which appeared in English in 1952.
"She starts with an imaginary person whom she is telling about sex, so she creates a kind of literary environment to write about a subject she's maybe not comfortable with", he said. The Anne Frank House said it used new "image processing technology" to decipher the covered pages.The Franks were caught by the Nazis in 1944.
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About prostitution, Anne wrote: "All men, if they are normal, go with women, women like that accost them on the street and then they go together". The Frank family lived in a modest brick building from 1934 in the Dutch capital, after fleeing rising anti-Semitism in neighbouring Germany, until they went into hiding. "Papa has been there".
The two newly revealed pages were written in Frank's first diary, with a red plaid cover, on September 28, 1942, when she was 13 years old. The family went into hiding in July 1942 and remained there, provided with food and other essentials by a close-knit group of helpers, until August 4, 1944, when they were discovered and ultimately deported to Auschwitz.
She died aged 15 at Bergen-Belson concentration camp in 1945.
Defending the decision to release the newly-found texts, the foundation said "over the decades Anne has grown to become the worldwide symbol of the Holocaust, and Anne the girl has increasingly faded into the background".
The Anne Frank House, a museum located in Frank's former hiding place, did not quote directly from the text it had recovered.
"Anne's diary texts show that she had gleaned information on the subject of sexuality from her parents, especially her father, from her friend Jacqueline and from books", the statement reads.