None of this is true. There is no evidence whatsoever that any exposure to pornography, depicting adults, real children, or fake children, encourages or leads to the abuse of children. At all. Zero. None. Don't make stuff up.
There is, however, the possibility that such desires could be (in some cases) satiated with a proxy, like porn. After all, there are plenty of guys who look at rape porn, but then don't go out and rape anyone. Their desires are fleeting, and satisfied with fantasy. Why would pedophiles be any different?
This part is actually true. And it makes sense. Even if you catch the perpetrator afterwards, they still stole someone's body parts and, you know... murdered them. So really, the problem is murderers. But at least the logic here is sound.
Those clean professionals are the norm in the counties where prostitution is legal. To suggest that "criminals" will "take over" a legal industry is to suggest that post offices are subject to the same problem. If a criminal element can infiltrate one legitimate business, they (the mafia?) can do the same for any industry. If an industry is above board, in the light, then it removes the criminal element. It doesn't increase it.