"It seems that POPU (Problematic Online Porn Usage) is not only one subtype of hypersexual disorder, but currently the most prevalent since it also frequently involves masturbation. Although this is difficult to accurately determine given the anonymity and accessibility factors that make pornography use today so pervasive, we can at least confirm that the patron of consumption for pornography has changed for roughly the last decade. ... As far as we know, a number of recent studies support this entity as an addiction with important clinical manifestations such as sexual dysfunction and psychosexual dissatisfaction. Most of the existing work is based off on similar research done on substance addicts, based on the hypothesis of online pornography as a ‘supranormal stimulus’ akin to an actual substance that, through continued consumption, can spark an addictive disorder. However, concepts like tolerance and abstinence are not yet clearly established enough to merit the labeling of addiction, and thus constitute a crucial part of future research. For the moment, a diagnostic entity encompassing out of control sexual behavior has been included in the ICD-11 due to its current clinical relevance, and it will surely be of use to address patients with these symptoms that ask clinicians for help."
Porn addiction has not been classified as an addiction in and of itself because researchers are unsure that it is its own addiction, or a subset of other sexual related addictions and disorders. So it is highly likely that porn is addictive, but medical professionals are hesitant to declare "porn addiction" as a diagnosis without seeing if it is separated from other problematic sexual behaviors.