Libraries are places of information storage and exchange, promoting free (well taxpayer funded) exchange of ideas being through any media (books, CDs, videos, etc.) By inhibiting the free exchange of ideas you are practically eliminating the whole point of the library.
First of all holding a librarian responsible for what patrons are viewing on computers is just dumb. Patrons should have the freedom to select any materials that are available at the time, by saying that librarians should be monitoring internet access like saying they should remove all the "bad words" from books or not having books that have any explicit material in them (ever look at the romance novel section?)
Additionally I know many libraries both public and university that have pornography in their collection, usually if you go into the periodicals section you'll find Playboy magazine.
I did IT support for a while and every once in a while we'd get some patrons (of age) coming in and looking at pornographic material. At the time I believe that there was some sort of rule in place that prohibited us from stopping him. I believe it came from the state library (this was in NJ.) I'm not sure if the rules have changed at all, but usually you're not allowed to censor material or put censoring software on computers (except of course for the children's area.)