SAN FRANCISCO >> On a recent morning at the main public library here, dozens of people sat and stood at computers, searching job-hunting sites, playing games, watching music videos. And some looked at naked pictures of men and women in full view of passers-by.

The library has been stung by complaints about the content, including explicit pornography, that some people watch in front of others. To address the issue, the library over the last six weeks has installed 18 computer monitors with plastic hoods so that only the person using the computer can see what is on the screen.

“It’s for their privacy, and for ours,” said Michelle Jeffers, the library spokeswoman. The library will also soon post warnings on the screens of all its 240 computers to remind people to be sensitive to other patrons — a solution it prefers to filtering or censoring images.

It is an issue playing out not just at libraries, but in cafes and gyms, on airplanes, trains and highways, and just about any other place where the explosion of computers, tablets and smartphones has given rise to a growing source of friction: public displays of pornographic content.

The subject can put personal media on a collision course with personal morality. This is an era, after all, that celebrates people’s ability to watch what they want, when they want, but it also forces bystanders to choose whether to shrug, object, or avert their eyes.

Some legislators battle against public displays of pornographic content, at least on the roadways. A bill is pending in the New Jersey legislature to criminalize the playing of obscene material in cars — say, on seat back DVD players or in party buses — that could viewed by, and distract or offend, others on the road. State Sen. Anthony Bucco, who sponsored the bill, said people who view such videos in public “don’t care what anybody around them thinks.”

Similar laws have passed in the last decade in Tennessee, Louisiana and Virginia, and one failed last year in Pennsylvania, according to the National Conference of State Legislatures.

An anti-pornography group, Morality in Media, has in recent months launched a “no porn on the plane” campaign, and has contacted most major airlines to argue that they should commit to policing what people watch.

The group took up the cause after its executive director, Dawn Hawkins, was on a flight in January and noticed a man in the row in front of her looking at images on his iPad of naked women whipping each other.

She complained to the flight attendant, who told her he was powerless to force the man to stop, she recalled. The man eventually turned off the images, but Hawkins continued to press him on why he was looking at those images in public.

She said a woman then came up to her and said, “Be quiet, nobody cares.”

“The fact of the matter is nobody did care,” Hawkins said. “I couldn’t believe people didn’t care that someone was watching pornography in public. I couldn’t believe society has come to this.”

For its part, Delta Air Lines says that it does not allow people to view “offensive content of any kind,” but also said that flight attendants are trained to make case-by-case assessments depending on circumstances and concerns of other passengers.

A spokeswoman for the Association of Flight Attendants said the issue is a bit of a “gray area,” handled on a case-by-case basis, adding that its members want to avoid offending passengers or playing the role of censors.

One reason the issue is so thorny is that not everyone agrees on what might be considered offensive. That is the case even within Morality in Media, where Hawkins said people should also be careful with public viewings of violent content.

But that’s not the view of the group’s president, Patrick Trueman, a former Justice Department official in charge of prosecuting child and adult pornography. “It’s not the same situation with violence,” he said, noting that graphic war scenes from a movie like “Saving Private Ryan” can provide a powerful history lesson.

Some people develop their own sliding scales for what is acceptable.

At Cafe Ponte in the Noe Valley neighborhood of San Francisco, the owner, Bruce Ponte, 53, regularly sees people looking at sites listing personal ads like Craigslist that can include naked photos. There is a parochial school next door, and he fears a parent will complain that he should restrict what people look at on their computers. He does not think he will.

“This is an Internet cafe,” he said. “People come here to surf. Am I supposed to do something about that?”

In fact, Ponte acknowledged that he himself sometimes sits at his cafe and surfs the hookup sites, and occasionally sees naked photos. But there are other times he takes a stand against what he considers a major public nuisance: patrons talking too loudly on their cellphones. He tells them to take it outside.

“They’re bothering everybody,” he said.

Ponte offers free Wi-Fi, like a growing number of cafes and restaurants. There have been a few reports of men being arrested over the last year for viewing pornography on their computers at McDonald’s. McDonald’s declined to comment for this article. Starbucks said it does not censor what people use its Wi-Fi for but reserves the right to ask someone not to view material that might offend patrons or employees.

Some people choose to act as their own censor. Lewis Goldberg, 42, a partner in an investor relations firm in New York, occasionally watches shows like “Mad Men” or “Game of Thrones” on his iPad when he works out at the gym. But he fast-forwards through sexual or particularly violent scenes.

“There’s a woman jogging behind me on the treadmill and I don’t want her to fall off,” he said. “I’m bringing my media into a public space, and it’s part of my responsibility in a civil society.”

Christi Chidester, 32, who works in communications for a federal agency in San Francisco, was in the main library here recently when she walked past someone watching hard-core pornography. She complained to the librarian and was told there was nothing that could be done.

Chidester said she has since spent a lot of time thinking about the issue, and while she wishes the library would create specific zones or rooms for those Internet users, she now likens seeing pornography on someone else’s screen to hearing someone curse in public. It’s going to happen sometimes.

Others fiercely defend the rights of people to watch whatever they want in public. When Hawkins, from Morality in Media, posted a YouTtube video describing her encounter on Delta, she was bombarded with angry emails from people telling her to mind her own business.

“People said, ‘just look away,”’ she recalled. “Their argument is that people can do what they want. This is America.”