My opinion:
Porn can be "addictive" because there is a significant emotional component to human sexuality and porn mostly gets a rise out of people by being shocking, which is why so much porn apparently revolves around social taboos (interracial, group scenes, barely legal in terms of age, etc). But the more you watch it, the more inured you become, so you need to move on to even more shocking things to get the same effect.
I think people suggest relationships as a solution because emotional intimacy can get that same strong emotional response without needing to keep upping the ante in an unhealthy way.
Do with that personal opinion what you will.
Best.