Libraries have copies of Playboy, Penthouse, etc. but they don't lend them out to minors. Libraries have never complained about this sort of censorship, but scream bloody murder over filtering out internet porn from our kids.
What is the problem with porn filtering computers used by children, and not filtering when used by an adult? Set aside one or two terminals within sight of the reference desk to monitor usage, and problem solved.
That makes perfect sense. But, I'm not so sure it would apply well in rural areas. My public library only has two computers that the public is allowed to use.