When Ryanair's chief executive, Michael O'Leary, announced recently that his airline may soon offer in-flight pornography, he told the British tabloid The Sun, "Hotels around the world have it, so why wouldn't we?"

The flaw in O'Leary's logic notwithstanding (hotel rooms have doors; airplane seats are surrounded by eyeballs, some very young), his proposal isn't so radical. As most any flight attendant will confirm, passengers are already indulging in racy content downloaded onto their phones, tablets or laptops from outside sources.

Beth Blair, a flight attendant and travel writer based in Minneapolis, said she once worked on a flight out of Burbank, Calif., during which an adult-film editor and his assistants began editing footage on their laptops. A child was sitting behind them.

"I asked them to turn it off ASAP," she wrote in an email. "Instead of obliging, they built a private area/tent out of newspapers. Luckily, the volume was turned down."

Extreme cases aside, the line circumscribing acceptable content is blurry. Pornography is one thing. But even a tasteful R-rated movie may have some scenes that parents don't want to have to explain to kids in neighboring seats.

The Department of Transportation imposes no rules on airlines in this regard, said Bill Mosley, a spokesman for the agency's Aviation Consumer Protection and Enforcement division.

Some airlines, such as Southwest and American, have chosen in recent years to block pornography sites from what passengers can access online. Many airlines give flight attendants no explicit rules about when they should intervene, leaving it to the crew's discretion.

Products like the 3M Privacy Screen Protectors, which render tablet and smartphone screens opaque when viewed from a side angle, could help reduce discomfort. Still, many passengers simply don't know how to behave because the rules are ill-defined and inconsistent, said Kate Hanni, executive director of FlyersRights.org, a consumer-advocacy group. When it comes to raunch, the problem seems simple and avoidable.

"If a child was going to a movie theater and the rating of the movie was such that that child couldn't get in, the same restrictions should apply on an aircraft," said Hanni, adding: "You can't count on people to be considerate."