The Web blackout mounted this week by Internet companies to protest anti-piracy legislation appears to have derailed the measures and won the day for an unusual constituency: online pornographers.

The industry calls them “tube” sites, a well-traveled red light district that includes PornHub.com, the 58th most viewed website among American Internet users, and XVideos.com, the 64th most viewed, according to Alexa, which monitors Web traffic. That’s on par with the traffic to Reddit, NFL.com and Hulu. Like many Internet companies, the tube sites are fiercely opposed to regulation and have been asking their visitors to urge lawmakers to oppose the legislation for months. Protected by the First Amendment, YouPorn.com and others voluntarily went dark Wednesday along with other, more mainstream sites like Wikipedia.

The tube sites’ pleas are bumping against the lifeblood of the industry: the professional producers of adult content like Vivid Entertainment and Hustler, the flagship publication of Larry Flynt’s pornography empire. They argue that they have suffered at the hands of piracy more than any other business.

“Sites in China and the European countries — their whole focus is plagiarizing. … I actually support some kind of censorship in that area. They are basically feeding off the American movie market,” Flynt told Roll Call in an interview Thursday. “But on the other hand, as a content provider on the Internet, I don’t want the government censoring what I do.”

The Stop Online Piracy and Protect IP acts would give the Justice Department the authority to block websites deemed to be illegally selling copyrighted goods such as movies, music and prescription drugs.

Steven Hirsch, the founder of Vivid Entertainment, said his company sends cease-and-desist letters on a daily basis to foreign pornography websites running his films without permission. But when the infringing sites are based overseas, he said, those letters are worthless.

Industry sources identified PornW.org, IntPorn.com and Kitty-Kats.com as the most common copyright violators. The illegal sites are often based in Hong Kong, the Netherlands and Germany.

Revenue for adult film producers has dropped by about 50 percent in the past four years, industry sources said. Many producers were slow to develop an online customer base even as the proliferation of free, often-pirated online videos crippled their DVD sales.

This week’s protests may have sealed the fate of the Stop Online Piracy Act, but the debate between content producers and disseminators will be on full display this weekend as the adult entertainment industry’s major players convene in Las Vegas for its most star-studded event, the Adult Video News Adult Entertainment Expo.

“No matter what the flaws are with the bills, the producers want something with a strong hand to come in,” said Allison Vivas, the president of Pink Visual, an adult entertainment company that specializes in mobile and Internet-based distribution. “We are a little bit more concerned. … When the government is responsible for enforcing these laws, there is always the concern that we in the adult industry will be overlooked. We don’t have the lobbying forces that [the movie and recording industry] have.”

Rep. Jared Polis (D-Colo.) tried to make SOPA worse for pornographers, introducing an amendment to the bill that would have prevented the Justice Department from using taxpayer dollars to protect pornographers’ copyrights by going after rogue sites. The amendment failed.

“He was concerned that SOPA’s main beneficiary would be companies that distribute pornography,” a Polis aide said in an e-mail to Roll Call. “The industry is known for being the biggest IP trolls out there and they’re more than capable of defending their IP claims without taxpayer assistance.”

Adult film producers said they have reached out to join forces with the Motion Picture Association of America, the movie industry trade group that aggressively lobbied for SOPA, but they have not been welcomed.

“A lot of those guys don’t want anything to do with our business; they look at us as absolute pariahs,” Hirsch said. “They probably all watch our movies.”

Enter the Free Speech Coalition, the trade association representing the adult entertainment industry. The organization works on behalf of both the content producers as well as the streaming websites and has been running an anti-piracy campaign for more than a year.

The Free Speech Coalition’s annual budget of about $500,000 pales in comparison with the MPAA, which spent more than $1.6 million on lobbying alone in 2010.

But copyright protection is a tough argument to sell in the porn universe. Vivid has struggled to convince customers that unless something is done about the piracy problem, the industry will produce fewer and fewer movies every year.

“People want things for free, and free trumps everything,” Hirsch said. “When it comes to adult content, some people only watch it for five or 10 minutes. It’s easy to pirate a clip and get it out there. And for some people that’s all the time they need.”

“It’s tough because you have the producers of content who want the piracy managed, but then you have the tube sites,” said the coalition’s membership director, Joanne Cachapero. “Many of those [people] come from IT backgrounds and feel very passionate about the freedom of the Internet.”

In ignoring the porn perspective, Hollywood may be missing some vital warning signs, the coalition’s chief lobbyist cautioned.

“The mainstream movie industry hasn’t experienced the kind of government overreach that our industry has, so we are much more reticent to just open our arms to any kind of government regulations,” said Diane Duke, a registered lobbyist for the organization. But she added, “We have no problem with going after tube sites that are violating copyright law with guns blazing.”

Steven T. Dennis contributed to this report.