Foucault, Satre, Derrida, Barthes literally went on record supporting sexual relations with children (see link below). The idea that something can be so valuable (like art) that it's worth the ethical costs elicited by their purveyors is ridiculous. Ethical behavior doesn't counter creativity (neither should the opposite be true).
I'm not sure that the value of the art was the supposed justification. Not an expert, but I thought they were just arguing for freedom, that children ought to be free to consent to things, not just that great artists deserved a break.
The ongoing Duhamel scandal makes it sound like there was quite a lot going on, in certain elite circles, everyone knew and nobody talked:
https://en.wikipedia.org/wiki/French_petition_against_age_of...