I initially posted it without the tag because that was my opinion too. But then I reconsidered in part because of the subject matter and the one image of the cover, but more broadly because a lack of an NSFW tag is one of those things that catch the attention of self-righteous prigs who are just looking for something to be offended over, and better safe than sorry.
The general lemmy audience is very prickly when it comes to what is or isn't NSFW content, especially in the art communities. In general, I think that the typical lemmy user is too sensitive about it (example). However, I appreciate playing it safe for this community though.
I am sure that with all the art he posts @[email protected] has fought the good fight against puritanical lemmy users many times by now.
It doesn't happen as much as you'd think. People like to point out that "hey someone might want this to be tagged" but I've yet to run into the people who actually do.