Note that I'm not saying that this would be a firable offense, but that someone could easily complain to HR about it and it could be considered creating a hostile work environment if it makes them feel unwelcome. These things matter!
Everyone's got an anecdote!
http://en.wikipedia.org/wiki/Hostile_environment_sexual_hara...
You'll notice 'pictures of pornography in cubicles' is one of them.
http://www.eeoc.gov/policy/docs/currentissues.html (a government website, note) mentions specific cases where visible pornography counted as establishing a hostile work environment.
And, an illustration in a browsed web article isn't the same as posted pictures in a cubicle.
Yes, the kind of hypersensitive context-oblivious standards you're describing exist in many workplaces. The point is they're not in all.
The dissenting comments and downvotes you've received contain valuable information: others' experience really, honestly, truly differs from your blanket claims.
Which is why it's good to have clear 'NSFW' warnings on things that might be inappropriate in the workplace. Trying to get really specific about what might be appropriate doesn't do anyone much good; just tag it NSFW if it's remotely questionable and people can check it out at home. Make sense?
Visible genitals - whether male or female - are often considered not appropriate for general audiences in the US. So it is not exactly crazy to assume that someone might be made uncomfortable or upset by a coworker viewing them at work.