I don't believe this is going on. I believe most sexism posts get flagged by users, not downmodded by moderators.
I strongly suspect that many users flag based on the title, without reading further. Most of the sexism focused pieces are pretty bad. I'm a woman and I tend to hate most of them.
Once or twice, I emailed the mods when I felt a post on the subject was poorly titled, but had worthwhile content. On at least one occasion, they changed the title and, I think, turned off flags for the piece so it could be seen.
I am careful to have a light hand. I seem to be the most prominent female member here and I can get accused of having an agenda for saying anything at all on the topic of sexism. *
I have tried to write more evenhanded pieces on one of my blogs. Some of those have gone okay, but they never really get all that much traffic or response on HN. But, as I see it, some of the problems with this topic include:
1) It has a long track record of ugliness on both sides, so some people kneejerk flag anything that sounds like it touches on the topic because they are tired of these toxic, pointless discussions.
2) Most pieces are pretty man-bashing. The audience here is predominantly male. They get tired of being hated on for having been born male. (I am also tired of the misandry, so I don't think that is unreasonable.)
3) Most pieces on the topic aren't really all that good. They are typically histrionic and blamey.
4) If you don't use the usual catch phrases in the title, people won't recognize that it is pertinent to a topic they are interested in. If you do, you can't sidestep all the baggage it involves. So trying to do something better faces a huge uphill battle in terms of even being recognized as both relevant and not guilty of the usual issues.
* https://news.ycombinator.com/item?id=15486833