I don't see why product integration could not be used in adult videos' production.
The problem might be that there aren't mainstream advertisers with enough balls to go for it, and advertising another adult product may create an issue with the videos licensees.
For example; licensee A may not be happy that you can see what appears to be a non competing adult product in the video they are streaming, because when you go to the product's website, you also have access to other contents offering that may be in direct competition with licensee A (As there is a high level of cross selling and up selling in our industry).
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