One of my favorite things to do in the library is to glance over the shoulders of porn-watchers and observe their reactions. I'd hate to have that taken away, but I can definitely see the drawbacks, and there'd be plenty of legal precedent to stopping it from happening. It would at least qualify as public lewdness or public display of offensive material I'd imagine.
Edit: Looked up some Washington law. Somebody's guilty of indecent exposure if "he or she intentionally makes any open and obscene exposure of his or her person or the person of another knowing that such conduct is likely to cause reasonable affront or alarm." Seems like this would fit pretty well.