Maybe there are some non-rapist men out there that are sexually attracted to mature women who just happen to LOOK like underage women. Presumably then such laws would be restricting the "adult entertainment" options of these men to have their every fantasy fulfilled in consumable, visual form. Maybe there are a lot of female gymnasts out there who have fallen on hard times and want to become adult film stars.
I would question, however, if these men are really devoid of pedophilic attractions, and if they have them, why would consuming such porn legally (or illegally) reduce the risk that they would act on these fantasies. A young woman, I can understand. But a woman who looks like she's pre-pubescent?
I guess I should clarify, I'm speaking more broadly than pictures of naked people here. To be frank, I'm talking about the hardcore stuff, recordings of actual sexual acts.
On another issue, these are automatically in a different category than say violent movies or video games (using violence as the comparison to the sex). Movies where the violence is faked with camera angles, choreography and even CGI.
Your typical porn movie (despite the passing of the 70's heyday of sexualized spoofs of popular films) is just a camera set up while people have sex. The key difference between this and prostitution is the camera.
We don't have to beat up any real people to create the latest "Mortal Kombat" game. We don't need to run over any real hookers or shoot any real cops to create the next GTA. Porn can be "faked" too, but generally speaking, we're talking about the real, even if it is created to cater to someone's "fantasy."
As for the data on the majority of women harboring fantasies about desiring to be raped, or credible studies showing a causal relationship between lax pornography laws (especially relating to porn depicting violation in a positive light) and lower incidences of rape... I'd like to see that.
The argument that rape porn REDUCES rape is another matter altogether. Is there any evidence of such a thing? Where does the demand for such a thing come from in the first place, and would this really function, society-wide as a cathartic measure? Presumably then, rape porn should be mandatory viewing for certain demographics, to reduce rape, if this is true, rather than prevention methods like self defense classes for women, teaching young men and boys that rape is wrong, ethical seminars against sexual harassment, selling pepper sprays, checking your drinks, etc.
Again, I didn't say porn CAUSED extra rapes to occur (say, turning "normal" people into rapists), I merely argued that laws restricting porn that caters to fantasies of criminal deviance involving real sex with people who LOOK underage are sensible. As for "inspiring" psychologically unbalanced people to commit violent crimes like rape (whether statutory or otherwise), of course, you can argue such a person might be set off by seeing an apple, but the most realistic impression that could be left on their mind, would be a depiction of the action itself, even if its staged between persons who are "really" over age.