People who had no clue what videogames were, were the ones arguing that playing a violent video game would make you want to commit actual violence. The counterargument that players made was that they could "tell reality from fiction" - i.e. that when they played Mortal Kombat or Call of Duty, they put their "Real Life" brain away and put on their "Fictional Video Game" brain, so videogames can't make people violent.

This is the right conclusion, but the logic is entirely wrong.

The reason why video games do not cause violence is that play violence is not anywhere close to the real thing, not that people firewall off fiction from reality. There's plenty of cases in which a piece of fiction has changed people's views! Crime shows are notorious for skewing how actual juries rule on cases. Perry Mason[0] taught them to expect dramatic confessions and CSI[1] taught them to weigh whiz-bang forensics over other kinds of evidence.

In the specific case of porn, there isn't really a difference between "play sex" and "real sex": they poke the same regions of your brain. And the people who are responsible for keeping actual pedophiles from reoffending are pretty much unanimous that the worst thing you can do is give them a bunch of, uh... let's call it "material". So if you're already a pedophile, giving you access to simulated CSAM won't substitute for the real thing. It'll just desensitize you to reoffending.

[0] https://en.wikipedia.org/wiki/Perry_Mason_syndrome

[1] https://en.wikipedia.org/wiki/CSI_effect