The general principle behind the specific argument you bring up here is this: All expression which is likely to inspire someone toward illegal action should itself be illegal
To me it’s more like “All situations where committing illegal actions could bring a positive feedback to the perpetrator should be avoided”.
Allowing CP to be shared, and thus sold/hosted on for-profit sites creates a market for it, and makes abusing children an actual profession. That’s not ok and already a talking point against the current, legal, porn industry.