You make a good point. If, in fact, illegalization of child porn does decrease the rape of children, then so be it....
But as I stated above, that path leads to ignoring mens rea. Instead of "did you mean to", it is "you possessed it, too bad". This is badness to the point that I could send people CP via picture message on their phones, and turn them in.
Or worst yet, a 15 yr old girl sent a naked picture to her boyfriend. And then, SHE was prosecuted for the production of child porn. And it's her own fucking body!
Lastly, I do not count on politicians (of any party) being reasonable about this issue. They make crap laws, with holes and failures, and the prosecutors apply said laws the way they see fit. So, I am forced to fight for legalization of the pictures (AND prison time for the rapists).