I don't remember where I saw this, but I came across two rules for civilized society.
1. Don't easily give offense.
2. Don't easily be offended.
It seems that your point leans heavily on rule two. We make determinations about what is acceptable in public spaces all the time. For example, we generally don't let people strip naked in them. In fact, if I go down to the library and strip naked in the public area, the police are going to come haul me away for public indecency. Is it really a big leap from there to having picture of naked people prominently displayed on the monitor? If someone was being insensitive to the people around them and talking loudly, it would be within the librarian's rights to ask them to quiet down or leave. Why not the same with gratuitous viewing of porn?
Yes, there are quite legitimate reasons for looking at nude pictures of people. But is it too much to ask that someone exercise a little bit of rule one? Tell the librarian you need to look at sensitive material. Use a kiosk in a private area.
I don't really buy the slippery slope argument, either. Not that long ago we had a guy strip naked in the security line at the airport to protest the invasive procedure. The judge acquitted him. His right to convey that message outweighed the issue of public exposure. We can have an ongoing debate that pulls us to a middle ground without succumbing to absolutism at either end of the spectrum.