Why are the trackers there in the first place? Most of the third-party code embedded in these websites is currently standard practice in the publishing industry. The New York Times embeds similar trackers and collects, uses and shares data about readers as part of its business practices. Some trackers , like those for Google Analytics, provide mundane traffic data to the site. DoubleClick and others provide the infrastructure to run advertising.

In exchange, these third-party companies receive data from the website’s visitors. Advertisers and platforms argue that this data is anonymous. And while some of it is basic (device type), other information (your I.P. address or your phone’s advertising identification number ) could be used to reverse engineer your identity and match you with already existing marketing profiles.

What these companies might be doing with pornography-site browsing data is a mystery. Oracle, which owns a number of large data brokers and has been called a “ privacy deathstar ,” could, for example add data collected by trackers to its current profiles. In the cases of Google and Facebook, which refuse to host pornographic sexual content on a number of their platforms, it’s not always clear why they are collecting such sensitive information, even if unintentionally.

Facebook and Google denied that potential information collected by their trackers on pornography websites was used for creating marketing profiles intended to advertise to individuals.

“We don’t allow Google Ads on websites with adult content and we prohibit personalized advertising and advertising profiles based on a user’s sexual interests or related activities online,” a Google spokeswoman wrote in a statement. “Additionally, tags for our ad services are never allowed to transmit personally identifiable information to Google.”

A Facebook spokes man offered a similar explanation, noting that the company’s community guidelines forbid sex websites to use the company’s tracking tools for business purposes like advertising. Though Facebook’s pixel tracker is open for any third party to install on its website — you don’t need permission to embed it — the company suggested it blocks pornography sites and, in those cases, does not collect information from those properties. The spokesman suggested that when alerted to new sex websites using the tools, the company will enforce against them.

Oracle did not respond to multiple requests for comment.

But even if the data is technically anonymous and not used for targeted ads, some browsing information may still end up in the company logs. And when it comes to pornography websites, the most basic browsing data is intensely personal because it is revealing. As Dr. Libert and Dr. Maris note in the study, nearly 45 percent of pornography site URLs “expose or strongly suggest the site content” and in doing so might reveal a visitor’s sexual identity or orientation, or lead third parties to assume a visitor’s sexual interests. “It can be very sensitive,” Dr. Maris said, citing URLs for specific interests like bestiality, and teenage and incest content.