Also, the main problem here, and it is a problem with other highly addictive behaviors like drug use, is that simply sticking with a so called "moderate" amount is nearly impossible. It is the whole point of things being addicting, becoming habituated to them, seeing diminishing satisfaction to previous amounts etc etc (for some people at least.. and I would guess this number is quite large).
Watch the video I linked above and see if it resonates with your own experience. Many long term porn users find themselves seeking out more extreme content over time, as well as an increasing quantity of porn viewed over time.
And what about the opportunity cost? What choices and behaviors are given up in the process of receiving sexual satisfaction from porn? Many data sources show a delay in marriage and declining quantity of sexual relationships or volume of sex for young people. Would anyone argue that this might also come with psychological costs in the long term?
“Moderate use of porn” doesn’t have to mean hardcore pornography. Playboy and erotic novels are probably not equivalent to Anal Whores 3.
Similarly, I’ve also heard the argument that high-speed internet pornography and “tube” sites in particular are the problem, because of how they present to the user a never-ending stream of novelty, and not necessarily because of the particular type of content.
If that’s the case then obviously the argument about moderation would have its merits.
It's possible to become addicted to much softer varieties of porn, but the potential ramifications of that addiction may be less severe (or they might be problematic in another sense, like financially).
For example an extension that tracks your porn viewing habits and limits video recommendations.
IOW “if X is in moderation, X is fine” is logically consistent with “some things cannot be in moderation”.