It's not strict Marxism leftism (Marx certainly was not a pedo apologist), but it's certainly the "woke leftism", or whatever you guys call it.
For them, the patriarchal family model promoted by Christianism in Europe was seen as oppressing to both women and children, with men having a monopoly on sexual matters. When feminism was all the rage in 1970s, it was part of a movement to liberate both women AND children sexuality from the tyranny of the head of the family (the man). The only reason they walked back the pedo crap very quickly is that the population made it crystal clear that they could liberate women as much as they wanted, but pedos would quickly end up at the wrong side of a rope.
Jack Lang, socialist minister of culture in France: "Child sexuality is still a forbidden continent. May explorers of the 21st century explore its coast."