I notice from the first discussion thread linked, evazion suggests that topless and shirtless should be separate because somebody searching for topless wouldn't want to see men. However, there are plenty of other tags where this is already the case. Given the site demographics, I'd wager most people searching for "masturbation" probably intend to find women rather than men with that search. Yet we still have an umbrella tag, masturbation, plus multiple gender-specific tags to narrow it down.
The one point mentioned to warrant making a distinction between "topless" and "masturbation" in terms of gender defaultism was that google searches return topless women by default, but I think this is pretty flimsy reasoning. While there may be a gendered bias in colloquial usage, in more precise usage (eg. most dictionaries) it is not identified as a gender-specific term, clearly many taggers are not viewing it as a gender-specific term by default, and tagging in general leans toward precision rather than colloquialism.
The umbrella solution seems abundantly common sense to me. A well-tagged image will often have 30+ general tags on it. Perhaps you can expect dedicated builders who have been at it for 20 years to memorize the particular rules of every single tag, but no casual contributor is going to be checking 30 wiki pages to make sure they're following every single rule to a tee, which means that tag names need to speak for themselves to a large degree. All the more so when the tagging rules are constantly changing on a patchwork basis with no larger unifying philosophy (ie. topless being for women only but masturbation being gender-agnostic is contradictory in a way that bucks any intuitive understanding of tagging rules). It seems there's been a tendency to blame taggers for not reading the wiki, but I think any "mistagging" as a result of the current situation is bluntly a policy failure, not a contributor failure.
