Probably moral depravity if I had to guess. Not sure why we even need “an argument” against it. It’s pretty self-evidently wrong.
Paedophilia is not synonym for child sex abuse, and to the extent that conjecture hasn't been ruled out by study already, that's at most an argument that its an appropriate function for tightly-controlled research-supporting tools, not a general public business.
Blatant disregard for children aside, the larger issue here is the presupposition that we should someone permit all urges. Not every thought that passes through your brain should be “explored”; some thing are just bad and should be fought.
Keep in mind that science can tell any story you want it to tell as long as you interpret the data in the way you need. That’s why some things, such as our children, cannot be left to slippery slopes such as this.
We’ve already got more than enough data proving that pornography is absolutely destructive and a net negative on all of humanity. I don’t think we need additional data to prove that bringing kids into the mix would somehow make it better.