Because time and again, sociological "research" that attempts to show porn as bad winds up being exposed as the worst kind of science, when it's even remotely related to science to begin with.

I don't like to be so biased based on my (dis)agreement with a study's conclusions. But in the case of porn research, it just so often winds up being about as close to relevant as "Reefer Madness" is to the effects of drug use.