I don't think people are concerned that people will get desensitized and not react to certain images or thoughts. The concern is that people will begin to reenact and recreate those things in real life.
If the "Offended" page, the amount of snuff, fecal porn, gore, torture and kitten videos are any indication, the internet has pretty much infected a lot of people with much more exposure to terrible things than any group of people prior. I can imagine that even the gladiatorial games of Rome pale in comparison to the volume, intensity and close-up nature of our internet.
And yet... where are the murders? Where are the feces eating lesbians? If we're desensitized, and many are, then why don't these things manifest themselves in real life? Because exposure does not make people commit what they see. If it did, we would literally live in Hell. Not sometime in the future, but right now, we would be living in Hell.