Although the term "postmodernism" is hard to define, it could loosely be characterized by it's anti-establishment, extreme social constructivist views. While there are exceptions, the tendency among these French intellectuals was towards anarchism and radical skepticism towards social norms perceived as bourgeois.
In the case of age of consent, I speculate the opposition came due the popularity of Wilhelm Reich's work The Mass Psychology of Fascism around the time of the May 68 protests in France. Reich had argued sexual repression starting in childhood was the root fascism:
Sharaf, Myron (1994). Fury on Earth: A Biography of Wilhelm ReichReich argued for sexualization of children via what he believed to be the Suppression of the natural sexuality in the child, particularly of its genital sexuality, makes the child apprehensive, shy, obedient, afraid of authority, good and adjusted in the authoritarian sense; it paralyzes the rebellious forces because any rebellion is laden with anxiety; it produces, by inhibiting sexual curiosity and sexual thinking in the child, a general inhibition of thinking and of critical faculties. In brief, the goal of sexual suppression is that of producing an individual who is adjusted to the authoritarian order and who will submit to it in spite of all misery and degradation. Initially, the child has to submit to the structure of the authoritarian miniature state, the family, which process makes it capable of later subordination to the general authoritarian system. The formation of the authoritarian structure takes place through the anchoring of sexual inhibition and anxiety.
It's understandable why MAPs would try to make use of the conceptual framework provided by these French philosophers for their activism. However, does this style of thought carry weight today? I'm skeptical. Consider the dissonance between the "postmodernist" Judith Butler's views (i.e. gender is a social construct that needs to be abolished) and those of modern trans rights activists. Seeking medical transitions and arguing that trans women are real women feel quite different from the spirit of gender abolition. Consider also how scientific realism with respect to climate change and vaccination have become a large part of what the progressive side of the culture war represents.
With this in mind, it seems even among the core political demographic that postmodernism appealed to, it's appeal has waned. People today are much more certain that what they believe is true, and much less agnostic than postmodernism requires people to be. Today, I think you have to fight for the objectivity of your own view, since saying all views are partial, biased, or subjective doesn't advance your position or actually diminish how strongly your opposition believes in their position.