Those people should face disciplinary action or Facebook should be blocked from work computers
Not every company has the same policy when it comes to computer use. Facebook is not blocked at my work and it's acceptable as long as it doesn't interfere with your work. It's pretty much accepted here that people will use their PC to check FB or their e-mail or something like that from time to time and no one really cares as long as you get your work done. Besides if you work at a place like that then you really don't need a NSFW tag on things because if it's a picture of your dog, you shouldn't be looking at it.
But the NSFW label is relevant for anything of a sexual nature, because if a coworker sees you on facebook or ESPN.com they won't sue the employer for sexual harassment. If you are looking a nude pictures they could sue your employer. And a coworker alleging sexual harassment will get you fired even if your boss turns a blind eye to you wasting time.
Nude pictures I can see as sexual harrassment. A picture of a birthday cake depicting a sex act I have a little more trouble seeing as sexual harrassment and I have seen pictures of things that were far less sexual in nature then either of these things listed as NSFW as well, like cartoons that merely hint at it being sexual in nature.
I think Spider is the most accurate here, better to be safe then sorry but the US has gotten far too prudish when it comes to sex for it's own good in my opinion. I don't think anyone would bat an eyelash over this cake in Europe.
Edit:
This is the definiton of sexual harrasment from the EEOC. Does a picture of a birthday cake depicting a sex act really fall under this description? I'm still of the opinion we have gotten carried away with the whole notion of what isn't safe for work, but that's just my opinion, I could be wrong.