So, for this thought experiment, can the illiterate people view movies, surf pornography on the internet, listen to audio books, purchase adult magazines, etc?
Don't forget that readers have access to all to these things as well. If written erotica has the negative effects that some say, there should be some evidence of this. The addition of this allegedly harmful written erotica should make the problem worse, as illiterates would not be reading it, whole readers would be reading it addition to all these other forms or erotica.
Also, we don't have to go back very far at all before these other forms of erotica were not available, thus the natural experiment is still valid. The internet is young, widespread access to internet porn has only been around for about 15 years. Easy access to porn movies only came about with the VCR, so that puts us back a little over 30 years.
As we go back in time, and keep removing technology, all we are left with is the written word. If written erotica turned people into sex offenders, we would see that readers were more likely to be sex offenders than readers. But we don't see this, not now, and not in the past.