It should be stigmatized.
Consent does not bless immoral acts or neutralize damage. A person who takes a drug voluntarily is still being harmed by it. It causes changes in the consumer whether he likes it or not. Causality does not care about your consent.
(And to address your analogy to books, the content of a work of fiction also matters. Reading bad books isn't good for your mind either. But literary fiction at least has the potential to be good. The genre isn't categorically bad.)
And porn is addictive. Porn addiction is extremely widespread and afflicts mostly young men. Porn's ubiquity and the easy with which it can be accessed has created a situation that did not exist before, and from a young age. And not only is it addictive, but it does real psychological damage to these consumers, creating what some call "porn brain". It is an excellent method for producing sexually-crippled creeps and incels unable - and even uninterested, given the nature of their "fantasy" - to have healthy relationships with real human beings, and the stats corroborate this.
It is an incredibly twisted and deranging vice. It destroys individuals and has a destructive impact on society as a whole.
> A person who takes a drug voluntarily is still being harmed by it. It causes changes in the consumer whether he likes it or not.
I’m afraid you’re oversimplifying it. If only things would be this simple. They just never are.
Every experience causes changes (it’s the whole point). And every stimulating experience has a potential to skew your behaviors towards having more of it. Some more, some less, of course, but anything can become a passion and get unhealthy so.
There’s this fine distinction between someone who does something now and then, without significant impairment to their decision-making abilities that cause over-favoring such actions, and those who fail to notice it in time and become overtly obsessed with something.
It’s not about what you do - you can be watching porn or going hiking (or whatever most people would naively deem “good”) - anything can become unhealthy.
I think I understand your point, though. Indeed, pornography consumption nature is intimate and that leaves less opportunities for feedback and self-introspection. That is, noticing the point it becomes more of an obsession. However, dismissing it under a simple “porn is bad” (a tempting idea) is short-sighted by dismissing any nuances, and also harmful - just in different ways (through stigmatization).