NSFW is not safe for the workplace. If you work in an office or with others, then an implied condition is that you behave appropriately when at the workplace. [A few workplaces will try and regulate your conduct _outside_ the workplace]. That may include not exposing workmates to content which could be considered questionable.
Many employers reserve the right to screen websites and prohibit some accesses - sometimes because of fears of additional reputational risk / security risks posed. That's a trade off - if you work in that industry, that's what you do during work hours and that' s an accepted part of gaining your salary.

In some sense, the flagging of something as NSFW is all the same as flagging when a previously innocuous domain name has been bought up by a gambling website / somewhere that serves porn / poses a risk of serving you malware which is considered routine good manners.