Collating some arguments people usually use in favor of CP. A lot of this is paraphrasing. These are just the most common ones that I have personally seen.
“The sex is consensual.”
They may not have the cognitive or sexual maturity needed to engage in it. And sometimes, they may do it purely as a desire for affection, even if they’re uncomfortable with it. You will also never know what is happening behind the scenes from a mere image / video.
“It is only nudes.”
This is admittedly better than abuse imagery, but it is still invading someone’s privacy and to some extent, causes them humiliation. This doesn’t mean that the mere existence of nude imagery is problematic (there are legitimate uses), but rather the distribution of it for sexual gratification.
“Some of it is legal.”
The law is a blunt instrument. It would unreasonable to ban everything, simply because someone could misuse it, just like it would be ludicrous to ban cars to prevent car related crime. Ultimately, it is up for the individual to make a call about whether it is truly moral.
“Most of it is self-produced and doesn’t involve an adult at all. It is consensual.”
They might be incapable of understanding the consequences of their actions, especially the idea that it might circulate far beyond where they intended it to and that it is far easier to post something on the internet than it is to take it down.
They may also find the distribution of such images to later be humiliating and to be judged for those actions by other people, even if they’re objectively the victims in that situation.
“It used to be legal and even accepted.”
A lot of things used to be legal. Sex with 10 year olds. Slavery. Marital rape. Society comes up with new standards all the time as the situation evolves.
“Criminalizing it causes harm to one group.”
That is why quite a few people are working on alternatives which don’t cause harm to anyone.
There is likely a lot more of these that I haven’t seen.