There are two problems with that: First, what is acceptable or not is a cultural thing and varies from country to country. There are actually countries where anything depicting humans or mentioning women is NSFW. In other countries they wonder "why has this been marked NSFW? They don't even f_ck!".

Second, even with atking this into account, and if we concentrate on an American and European context which has mostly comparable ideas of morality, people still mark things as NSFW where I ask "Why?", and other posts are not marked NSFW where I also ask "Why?" - if you understand what I mean.

So any usage of an NSFW tag has a certain ambuguity to it, and can only seen as a hint, as a personal thing to use or not use as the poster evaluates it.