No medium is more immediate than the Internet. No content more self-gratifying than porn.

So it should be no surprise that the meeting of smut and the web was a deeply disruptive event: the message had finally found its medium. Online porn made stars of young entrepreneurs like Seth Warshavsky, and ruined dead-tree old-timers like Penthouse publisher Bob Guccionne.

So why is mobile porn, which puts immediate gratification, literally, in the palm of your hand, such a business failure in much of the world? The problem, it seems, isn’t a lack of eyeballs. Adult content already accounts for half of all data traffic on U.S. wireless carriers outside their own portals.

The trick is getting paid—and oddly enough, that’s where the cultural problems begin. Luc Prieur, the owner of Phonebox Entertainment, which runs wireless porn site PhoneErotica.com as well as a niche gay site called BoogieBoys.com, claims he has not made a cent in the U.S. despite his site’s popularity. U.S. carriers are happy to charge for the airtime, he says, but not for premium content on adult sites. In this way, they are much like carriers in Islamic countries.

In the U.S., mobile carriers don’t want their brands associated with graphic sex. As a result, while mobile porn is a billion-dollar-a-year business, thanks to markets like Europe where mores are more liberal, it has stagnated in the U.S., which has made whole industries out of porn geared toward the Internet and video. Adi McAbian, CEO of Waat Media Wireless Entertainment, which provides content from such brand names as Playgirl and Vivid Video, claims his company’s products are available in 17 markets through 30 carriers. Not in the U.S., though. “In England, a topless woman on Page 3 would be acceptable” he says. “But to many people here, Maxim or Victoria’s Secret may constitute adult content.”

Standards must be introduced that will allow carriers to keep mobile porn away from kiddies. That’s coming, slowly. The Cellular Telecommunications & Internet Association (CTIA) has set up an action team to address the issue of mobile downloads. It met for the first time last January to thrash out guidelines for mobile carriers with respect to the delivery of content—such as a rating system that can identify smut.

The United Kingdom was the first country to develop a self-regulatory code of conduct for mobile operators offering adult content, a category that includes porn, gambling, and violent games. The code, which went into effect late last year, calls for an independent body to determine whether content is unsuitable for mobile customers under age 18. Any content restricted to those over 18 is not supposed to be made available until operators verify a customer’s age.

Once the smut can be kept away from kids, though, content providers will need a way to get paid. Phonebox’ Mr. Prieur says users prefer carrier billing to credit card billing. Only 5 percent of users are willing to use their credit card, he says, while 30 percent would rather put their charge on their phone bill. For carrier billing to come about, though, carriers need to become comfortable with the concept of adult content, which may never happen in conservative societies.

The solution could be some sort of hybrid. On July 2, Vodafone announced the launch of Content Control, a barring and filtering mechanism which will prevent underage customers from accessing adult content. The system would automatically enroll new customers in the program, which will block them from accessing 18-plus-rated content until they register via the web, wireless device, or in person. This is a great example for other carriers, getting people to “opt in” to age-restricted content, as opposed to providing them that content and letting them “opt out” when they do not want it, says the Yankee Group’s Mr. Zawel.

Once such solutions are in place, mobile porn will get a start on fulfilling the promise of an earlier generation of online smut.Mr. McAbian says that with Fortune 500 companies like DirecTV, Hilton, and Time Warner offering mainstream respectability to pornography, operators won’t stay away from the business for much longer. As a result, the mobile adult content market in the U.S. should grow to $100 million in 2008, says the Yankee Group. “Mobile carriers can’t keep their heads in the sand forever,” says Mr. Zawel. Don’t count on U.S. carriers to pass up a cheap-and-easy boost to profits from mobile porn for much longer.