It's a large, smelly, mangy dog chasing its own filth-encrusted tail.
I see the argument you described as more of a large straw man you would like to burn at the stake.
May I put some words into your mouth? You put an awful lot in to mine so I feel free to return the favour.
A: "What's the evidence child pornography causes harm?".
B: "It must cause harm! Underage sex is bad!".
A: "Actual evidence indicates that sometimes it is, sometimes it isn't, depending on the cirumstances. But let's get back to the original issue, fictional depictions of underage sex".
B: (fingers in ears) "Underage sex is badbadbad always! This one person had underage sex and it was terrible for them!".
A: "These eight hundred people filled out surveys and they had underage sex and sometimes it was bad and sometimes it was not, depending on the cirumstances. But let's get back to the original issue, fictional depictions of underage sex".
B: (fingers in ears) "We know underage sex is bad because we have all sorts of purely theoretical arguments! People under X, where X is the local age of consent, cannot consent, and are at risk of exploitation, and they don't need to have sex, and they're emotionally unready for it, and they might catch STDs!"
A: "If that were actually the case surely it would be terribly easy to find evidence of this harm in epidemiological studies? Although we're still a long way off the point, which was regulating fictional depictions of underage sex".
B: "Evidence? How dare you ask us for evidence? Firstly our position is self-evidently true as demonstrated by our purely theoretical arguments. Secondly we have anecdotes, which are better than large surveys. Thirdly we are parroting current cultural values, which exempts us from ever needing evidence. We have the right to clog the thread with vile accusations and fantasies of extreme violence as much as we like, and we need never even look for actual evidence".
A: "So you're never going to discuss the actual evidence you have been presented with?".
B: (fingers in ears again) "You're a bad person!"
A: "Are you going to address the awkward issue that the definition of underage varies from eleven to nineteen depending on location and situation, and that you have no evidence any one age is better than any other? Thus activities you violently condemn in some jurisdictions are held harmless in other jurisdictions?"
B: (fingers in ears) "You never post any rational arguments and you ignore all of my rational arguments. That's why this thread is not going anywhere."
A: "So you're going to ignore the original issue, fictional depictions of underage sex?".
B: "Of course we are. How are we supposed to work ourselves into fits of self-righteous rage about that stuff?".