I agree, I use the nsfw tag currently. But because I had to do some research to find out what tag to use (and that indeed a tag existed) I propose a check-box in the post form so there is no ambiguity and posters can not claim they didn't know of the option.
With your suggestion, that would mean two check-boxes, one for nsfw and one for nsfc.
Do you mean something like this where you go to write the post?
yes exactly!