Would you ask someone in HR this question, if you were in an elevator with them? Would you bring this up with your parents or some kids?
Some smell tests I'd use for determining if something is not safe for work.
Quite frankly marking something that could be NSFW isn't about you. It's a courtesy to others who could view it is nsfw. You're not required to be courteous, of course.
That's a copout and you know it lol. If you wouldn't discuss this with HR, it's a good sign that it's NSFW.
This is about whether or not the content is nsfw, not about whether or not folks should see this. Marking nsfw content as nsfw is not censorship, nor does it prevent people who want to see it from seeing it.