Here's a 21st century etiquette tip: when you're at the library, don't peek at your neighbor's computer screen--it's rude.

Also, you might accidentally see people having sex.

The San Francisco Public Library is increasingly becoming ground zero for porn-viewing, and library officials have decided to do something about it.

No, they're not outlawing adult viewing. They're putting up plastic privacy hoods around some of the library's monitors so passersby don’t accidentally catch an unintended glimpse of some porn star's butt.

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Over the past few weeks, the San Francisco Public Library has installed 18 privacy screens on computer terminals inside its main Civic Center branch.

"We're always looking for any kind of elegant solution that strikes a balance between the right to privacy and folks that want to use the library for any other intended purpose," librarian Luis Herrera told KTVU.

The new screens are installed as a pilot program, and their number will increase if they prove popular with patrons.

Library officials told the New York Times they plan on posting warnings on every one of their 240 computer screens urging users to be respectful of other visitors when deciding which websites to view.

"It's for their privacy, and for ours," library spokesperson Michelle Jeffers explained to the Times.

The San Francisco Public Library's conundrum represents a tension libraries around the country feel between most librarians' natural aversion to censorship a desire to make their buildings a comfortable place for everyone.

New York Times ethicist Randy Cohen opined:

Libraries should provide for the free exchange of ideas -- not just ideas you or I find palatable, not just ideas suitable for 5-year-olds. And librarians should not be forced to censor patrons' reading, let alone eject them for looking at disturbing images.

In 2003, the United States Supreme Court issued a ruling that required public libraries to install filters blocking adult content order to be eligible to receive federal funds for online education. Each individual library has the ability to choose exactly how those filters are applied--most can be turned off with the click of the mouse.

“A public library has traditionally and historically enjoyed broad discretion to select materials to add to its collection of printed materials for its patrons’ use," wrote the court in its majority opinion. "We conclude that the same discretion must be afforded a public library to choose what materials from millions of Internet sites it will add to its collection and make available to its patrons."

However, in cases where the filters can't be turned off, there's still some legal controversy as to what precisely can be blocked.

Earlier this year, the American Civil Liberties Union filed suit against a library in Washington State whose block on porn also excluded research sites and certain political content.

"Libraries should not limit the opportunity of adults to view research articles and other lawful materials," ACLU of Washington legal director Sarah Dunne said in a statement to the International Business Times.

Most libraries nationally take a similar view as San Francisco and see pornography, at least to some degree, as protected speech. Instead of outright bans, many take steps to merely dissuade its viewing.