As a new mod, I think a good way to judge is to try to image the reader (a teacher) is in a classroom with 10 year old children. If the headmaster or a child were to look over their shoulder would they see anything in the question that might give them reason to complain?
It is a judgement call. I have a very dirty mind so the word fingering caught me out. I asked someone to change that. It was pointed out to me, fingering could mean playing a guitar and of course that is true. :-) If in doubt, we can always ask other mods and if you disagree, you can always bring it to our attention.
Since I am actually very liberal, I probably go too far the other way at times in terms of trying to gauge ‘what could be construed as offensive’.
I do think the NSFW tag is appropriate and sufficient. It is a warning that the content may not be for all readers and if you progress from there, you have accepted that risk. Putting it in full probably wouldn’t be a bad idea and it would remove any possibility a person could open the question accidentally. I do think though, once open, most people can judge if the content is for them or not. If it isn’t, close it and move on.