News site TorrentFreak found some sites had uploaded videos that did not show up on YouTube, but could be viewed on third-party websites.
The exploit allows a website to host its video library for free, using Google's server space and bandwidth.
YouTube told the BBC its policies "prohibit sexually explicit content like pornography".
Videos can be uploaded to YouTube under a "private" setting that prevents them from appearing publicly on the website or in search results. This setting also disables the embed function that usually lets videos be posted on other websites.
However, TorrentFreak reported that some websites had found a way to play secretly uploaded videos on their own external services, by streaming the raw data from googlevideo.com - a domain operated by Google.
The news site said it was not clear exactly how the websites were achieving this.
Hosting videos on YouTube secretly would let an adult video site keep its costs low, while earning money selling access to its videos.
One California-based adult film producer suggested that the loophole was also being used to host pirated adult content.
"Copyright infringers take advantage of a private-video-share setting," Dreamroom Productions told TorrentFreak.
"They upload and store videos, and freely use them on third party websites to earn profits."
The company said Google did take down infringing copies of its content when notified, but added that the process sometimes took up to three weeks.
"YouTube should be aware of this. They are allowing the situation to continue by not plugging this hole," the firm said.
A spokeswoman for YouTube said: "We have teams around the world that review flagged content, regardless of whether it is private, public or unlisted. Content that violates our policies is quickly removed."
Adult video websites appear to be exploiting a YouTube loophole to host explicit material on the platform.