People who regularly watch porn will tend to become desensitized to it. This will push them to seek out more extreme, taboo kinds of porn in order to reach the same level of arousal, creating a viscious cycle which will, if left unchecked, eventually lead to serious crimes (eg rape).
I take issue with this reasoning. I think there are a couple of real processes which are being misunderstood and conflated.
The hidden assumption is that more "extreme" kinds of porn (the kind associated with criminal activity) are a natural "next step" from vanilla. As though rape porn is simply a more effective version of vanilla porn, and that naturally, desensitization will cause one to progress in that direction. For this to be the case presupposes that the person already has a preference for this more extreme genre.
Compare to, for example, coffee addiction. As one gets in the habit of drinking coffee, one develops a caffeine tolerance, so the coffee is less effective. Naturally, this will push one to experiment with more niche, taboo, or "extreme" kinds of coffee in order to get the same effect, right? No. That is silly, and not how addictions work. You just drink more coffee. Likewise, your 100th porn experience will probably not be as intense as your 1st. This will most likely just result in you watching more porn, not to shift you to the genre next in line on some arbitrarily concocted ladder of extremification.
We can't really take this for granted, though... Not everything that causes pleasure will result in this sort of addictive amplification process. You probably eat food when you are hungry, and then stop when you are full. There is not necessarily a self-intensifying cycle of needing to eat more and more food each day to quench your hunger. Food addiction does exist, and so does porn addiction. But just as eating tends to simply be a natural, nonaddictive release of hunger, I think that porn use can serve as a similarly nonaddictive release of sexual arousal.
That's real process #1; #2, is just novelty-seeking. As one engages more with a medium, one will naturally become curious about some of its niches. You'll get tired of looking at the same sort of thing over and over, so you'll eventually want to expand your horizons, broaden your taste, try something new. In exploring a medium beyond the mainstream, one is likely to find some particular niche which speaks more deeply to them than the works with a broad appeal. This results in a kind of specialization, where everyone finds their own special "fetishistic" sorts of art that appeal specifically to them. This is simply called "developing taste", and it applies to pornographic art the same as any other.
Anyone who thinks there is any sort of universal straightforward progression of increasingly "extreme" porn (as opposed to a multiplicity of bizarre niches) has clearly never been on /d/, and their opinion on the subject should thus not be taken seriously.