While no one is willing to project specific financial losses, most agree that the company most likely to be affected by these restrictions is Manwin, which controls the Brazzers and RealityKings collection of websites as well as several adult "tube" sites (think Youtube for porn).

"It will be very easy to block these tube sites," said Ogas. "Those account for the vast majority of people's online porn use."

U.S.-based porn companies say they're bracing for a potential hit, but won't know how big a disruption it will be until a few details about the opt-in process for viewing adult content online are made clear.

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"I'm not sure what you're going to see when you go to those sites now," said Michael Klein, president of LFP Inc., which owns and oversees the Hustler empire. "The only sites you're affecting are the ones who already have [age verification] precautions in place. ... When you type in Hustler.com, are you going to see a page saying 'enter your credit card'? [If so,] that's going to have an impact because people have no idea what they're buying. ... The U.K. is just one portion of our business, but I'm sure it will have some impact."

Adult holders of existing accounts will be asked directly by the ISPs if they'd like the filters added to their accounts over the next 18 months, the government said. That, essentially, will require customers to not only verify their age, but also confirm that they'd like the ability to watch porn online.

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And having to make that declaration could be costly for the industry.

"Obviously people are not going to want to do that," said Robert Rosen, a porn publishing veteran and author of "Beaver Street: A History of Modern Pornography". "People just don't want to come out in public and say 'I want to look at porn'. A lot of people who do look at porn are inhibited, shy people."

The U.K. is hardly the first country to ban online porn. Last year, an Egyptian court ordered that all X-rated websites be blocked. China and Saudi Arabia also have mandated filters on their Internet connections. Officials in Iceland, a largely liberal country, proposed a ban earlier this year—though the movement seems to have lost steam after the country's ruling party lost control in recent elections.

And in India, ISPs were ordered to block 39 websites serving up porn earlier this month—following a petition from the country's Supreme Court to establish a ban on online porn. (Government officials stopped short of a complete ban, saying they didn't believe such a thing was possible.)

That may, in fact, be an accurate assessment. Porn, like nature, tends to find a way.

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While Cameron warns that access to online porn is "corroding childhood," Rosen notes that children have always found a way to circumvent rules meant for their protection.

"If kids want to look at pornography, they usually figure out how to do it," he said.

Ogas agrees. For "A Billion Wicked Thoughts," he and co-author and fellow neuroscientist Sai Gaddam did an extensive study of online pornography to determine how the area of the brain that generates sexual desire and arousal worked—and were amazed at how widespread it was.

"I can tell you from our research, we went through and tried to identify every piece of porn online—and it is an impossible task to try to filter it all out," he said.

—By Chris Morris, for CNBC.com.