They are probably thinking of Beauvoir and Sartre in particular. Good luck finding a scholarly source on the issue though, unless you're quite happy with citing A Voice For Men or random angry blogs.
Neither Sartre nor de Beauvoir where postmodernists (or "pot-modernists"). Poster maybe means some radical left anarchists – also not postmodernists – from the late 70s/early 80s, who had some outlandish (and completely self-serving) ideas about child sexuality.
Yea I know. Postmodern is a snarl world popular amongst certain demographics overrepresented on HN.
Foucault, who also signed the letters, is often considered a post-modernist, even if he himself rejected the title.
Beauvoir and Sartre had reputation for seducing their students, but not when they were underage (for France's laws of the time).
No need for it, they published an (two actually) open letter defending sex with children as young as 6:
"the children have not been victims of the slightest violence, but have to the contrary testified before the examining magistrates that they consented"