I'm sure people have different preferences within this category, and the purpose of tags is to enable searches and filtering, right? That seems to me to imply that such tags could be a useful thing to have.
To me, the ideal system would be sub-tagging. This would allow you to tag something as "Incest" and be done, if you like. That would default to "Incest: Unspecified". But you could also add the related subtag(s), if desired. Then people could search/filter at least to the extent that the data is complete.
Then, as a viewer, you could also search/view/filter only only by the main tags, or you could use the more specific sub-tags. You'd want good rules regarding the default applied subtags and search/filter interactions, but it could very easily be all the benefits of both systems depending purely on user choice. While browsing you can see the more condensed information of only the main tags, but if you look into them farther or are performing a search you can also make use of the sub-tags to get more specific with your preferences.
The same system could be useful for all sorts of tags, too, not just this one - the NTR discussion would be a good application, the urination someone mentioned above, etc.