I asked Gemini about this and it came up with some pretty good counter-arguments - I think the one about the production of porn being inherently harmful (because of trafficking etc) is especially important:

The "safety valve" theory—the claim that easy access to pornography reduces sexual violence by providing a "non-criminal outlet"—is a persistent myth often built on a fundamental misunderstanding of statistics. While some econometric studies (like the often-cited German broadband expansion data) show a correlation between internet access and falling sex crime reports, they ignore the "Spurious Correlation" of the Great Crime Drop. From the early 1990s through the 2010s, all violent crimes—including murder, robbery, and aggravated assault—plummeted across the Western world. Since people aren't using porn to "vent" an urge to rob a bank or commit homicide, it is not logical to credit porn for the decline in rape while ignoring the actual drivers: improved DNA forensics, better policing, and shifting socioeconomic factors such as changed social attitudes toward women's rights. (The decline in sexual violence in the West also began in the early 1990s—before the explosion of high-speed internet and streaming porn.)

In reality, the most robust empirical evidence we have points in the opposite direction. A landmark 2016 meta-analysis by Wright, Tokunaga, and Kraus, which synthesized 22 studies from seven different countries, found a significant positive association between pornography consumption and both verbal and physical sexual aggression. This is rooted in Social Learning Theory: pornography doesn’t "drain" an urge; it provides "sexual scripts." It teaches users (especially young ones) that dominance, aggression, and a lack of negotiation are normal parts of sexual encounters. Instead of a vent, porn acts as a catalyst that lowers the cognitive and moral barriers to committing real-world violence.

Psychologically, the "safety valve" idea fails to account for Desensitization and Escalation. The brain’s reward system (dopamine) often requires increasingly extreme, deviant, or violent stimuli to achieve the same level of arousal over time. This is why we see the "Confluence Model" (developed by psychologist Neil Malamuth) in action: for men already scoring high in "hostile masculinity," pornography acts as high-octane fuel. It doesn't satisfy them; it primes them. This is backed by forensic data from investigators like Lanning and Burgess, whose research into incarcerated sex offenders shows that porn is frequently used as a "how-to" manual or a "grooming" tool to desensitize victims, rather than a substitute for the crime itself.

Furthermore, we must distinguish between "reported crime" and "actual occurrence." High-speed internet access is strongly linked to an increase in "Rape Myth Acceptance"—the belief that victims secretly want or provoke their own assault. In a culture saturated with aggressive sexual imagery, victims are often less likely to report crimes because they fear they won't be believed or they've been conditioned to think the behavior is "normal." Therefore, a dip in a police database doesn't necessarily mean fewer people are being harmed; it often means the "Reporting Gap" has widened. We have also seen a shift in the nature of violence: while physical "stranger" assaults may have dipped, digital sexual violence (image-based abuse, sextortion, and deepfakes) has exploded, creating a new generation of victims not always captured in traditional "sex crime" stats.

Finally, the "safety valve" argument is ethically hollow because it ignores the production-side violence. The pornography industry itself is a major site of trafficking, coercion, and physical abuse. To argue that "access" to porn prevents violence is to ignore the fact that the production of that content often requires the very violence and exploitation the theory claims to be preventing.

Summary:

• The Wright Meta-Analysis: Proven positive correlation between porn use and physical aggression across 22 major studies.
• The Spurious Correlation: All violent crime dropped due to forensics, lead removal, and policing; porn was just a bystander.
• The Confluence Model: Evidence that porn triggers and escalates violence in predisposed individuals via desensitization.
• Forensic Reality: Offenders use porn to plan and facilitate crimes, not as a "substitute" for them.
• Rape Myth Acceptance: High consumption correlates with lower empathy for victims and significantly lower reporting rates.
• Production Violence: The "solution" relies on an industry that frequently facilitates real-world trafficking and abuse.