I am going to throw out a few ideas here just for discussion.
A) Large sectors of the Left have always found biology and the body problematic. It, the body, somehow did not fit in neatly with utopian schemes. For many, it was a distraction from the urgent matter of "building a revolution" (what kind of revoltion without the body?), hence sexuality, especially erotic sexuality was seen as a distraction, a bourgeois deviation. This part of the Left does not see the fulfillment of human desires as part of the revolutionary agenda.
More generally, much of the Left has historically distrusted the topic of biology. They are stuck in rationalism...any discussion of genetics raises the spectre of fascism in their eyes. No, better abandon biology to the Right.
B) As part of its general retreat, much of the Left has abandoned any positive demands. Instead, many prefer the safe territory of identity politics where they can be victims and/or framing everything in moralistic terms. Raising a radical demand, like greater sexual freedom would be too risqe'.
===== Unlike its unruly city counterpart, the suburban body has been wholly domesticated and one can say that the suburbs consitute a huge petting zoo, with the residents' bodies providing the stock of furry mammals. -JG Ballard "Project for a Glossary of the Twentieth Century"
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