I think porn addiction is something real people struggle with.

A definition of an addiction may be in order:
I believe an addiction is an overwhelming compultion to repetitively and frequently revisit a gratifying behaviour that is percieved as harmful to one's life, morally, socially, relationally and/or physically.

There are many overwhelming compultions we have that are not addictions... breathing... eating.... sexual release... craving for the company of people... thirst for water... etc...
So the harmful part of the definition is very important to my view.

In this respect.... not all porn lookering will be addictive behaviour.
Some people may feel that they can look at porn in a casual, take it or leave it, kind of way; they are not overwhelmingly compelled to look at it.
Others may feel that their porn lookering is not harmful to their life (and they could be correct or they could be in denial).
Others are very aware that they have little self-control when it comes to looking and that their looking is effecting their job, thier relationships and their general health etc... (for example... if one is staying up all night looking at porn and not getting sleep). This kind of person often wants to stop but feels powerless to do so. This is the classically addicted person.

Having said that porn addiciton exists however, I don't automatically believe that it degenerates in degrees.
If a person starts out on soft stuff and progresses up to harder material, my suspition would be that they always fancied the harder material to begin with but just didn't know where to get it (and became more practiced with time) or they were morally resisting their own current desires (and over time they just lowered their resistence). In such a case the person when viewing the softer stuff would be saying, "this is good but what I really want to see is..."
There is a point of saturation where looking at something becomes boring and ungratifying.... but the solution to this is not harder stuff, rather it is to have a break and come back later when your hunger is refreshed. We are cyclical beings who go from periods of hunger to satiation and rest and back to hunger. The desensitising of neruo transmitters that makes some narcotic drug addictions require heavier and heavier fixes to get the same buzz is not true of sexual gratification I believe.

Blessings
Cat.