In the late 1970's there was a letter writing campaign published in newspapers like Le Monde supporting the release of men accused of sexual relations with 13 to 14 year olds. https://fr.m.wikipedia.org/wiki/Pétitions_en_France_concernant_la_majorité_sexuelle Signing the letter are big names from Simone de Beauvoir, Jean paul Sartre, Francais Derrida, and Micheal Foucault. Why did they sign this letter, and why was there support enough to publish something like this in a newspaper like Le Monde?
About the question in your post:
The full translated text of the petition is available here. That was not a solely existentialist list, many intellectuals of the time were there.
As you can see from the text, these people saw an incoherency in the laws that recognized 13 years olds enough discernment to be jailed for their actions, that accepted to prescribe the contraceptive pill to them but refused to recognize their ability to give consent to sexual acts.
This was a part of a legal debate of the time that was looking at a reformation of the laws surrounding sexual morality and consent, in the aftermath of the sexual revolution.
In the 70s France, as well as in a big part of the western world, was undergoing the sexual revolution. Many reasons for it : better gender equality, contraceptives, lower religiosity... People were exploring the limits of sexuality once the old moral constraints were abolished.
People started talking about sex before marriage, about homosexuality and started admitting that people younger than 21 (the then age of majority, it would be lowered at 18 in 1974) could have an active sex life with consensual acts. How young? People in the letter you write argue that you can be younger than 15 and consent. They argue that if a 13 year old can be judged and go to prison then they should be able to also consent to sex.
Thing is, it took some time to appreciate that consensual sex between two 14 years olds was a very different relationship than between a 30 year old and a 14 year old.