Kühn, S., & Gallinat, J. (2014). Brain Structure and Functional Connectivity Associated With Pornography Consumption: The Brain on Porn (https://jamanetwork.com/journals/jamapsychiatry/fullarticle/1874574)

This is the classic "correlation does not equal causation".

The study quite openly says that they can't tell from their study whether the differences they found were caused by pornography viewing, or whether they have discovered a precondition that makes someone more likely to enjoy watching pornography.

In addition, it's unclear exactly what a reduction of grey matter in the right caudate means in real world terms. Body size correlates with grey matter volume, and no one would claim that a smaller person, with a lower volume of grey matter has a "destroyed brain".

Loss of existing gray matter in that area may not be benign, but we don't know, because the study doesn't explore that topic.

Love, T., Laier, C., Brand, M., Hatch, L., & Hajela, R. (2015). Neuroscience of Internet Pornography Addiction: A Review and Update

Hilton, D. L., & Watts, C. (2011). Pornography Addiction: A Neuroscience Perspective

These two articles talk about pornography addiction, which is distinct from pornography consumption. In addition, both of them talk about the harm caused by pornography addiction as being a result of addiction, rather than an inherent aspect of viewing pornography.