I agree with Walter that this can be a two-edged sword, further complicated by the fact that the process would be relying on image posters having the integrity to properly rate their images. In general, when I direct someone to my galleries, I supply a URL to take them right to the gallery I want to show them. Beyond that, I have no more control over what they look at any more than I have control over content posted by others. If a viewer chooses to explore, I don't see that it's my fault if they stumble across something that upsets them.
In a quick look at help, I don't see a way to Ignore/Block specific users (but I'm vaguely recollecting there might be one). If that is or could be made available, one could at least make some of the dirtballs invisible to one's self. Like Walter, I have seen at least one site full of racist, political garbage that I find far more offensive than any human body. Whenever I see the word "porn" used, I begin to worry, as it seems, depending on the viewer who is complaining, the meaning of that word can range from explicit sex acts (which I agree we don't need here) down to showing a little ankle!
In the galleries on one forum I belong to, there is a little icon under posted images (and even posts, for that matter) to report the object to a moderator. I suppose something of that sort with a working version of NSFW could be used to allow users to monitor NSFW compliance, but it would presumably require moderators which we don't currently appear to have. And of course, this gets us back to someone somewhere making some sort of value judgement on what constitutes over the line.
There's an old saying: "If God intended us to be seen naked we'd be born without clothes on...."
"Oh --- nevermind!"
DaveT