There doesn't need to a violent result to justify criminalization. Laws are about how a nation defines behaviour and creates community. Like I said - I have no idea whether it causes sex crimes or whether there's any meaningful corollary at all. I do think it's likely that exposing children to rape images with the purpose of sexually stimulating them is likely to generate those fetishes. But, in part that's not the point.
It is perfectly valid for laws to work toward the betterment of society by discouraging attitudes and behaviours that are self-destructive or counter-productive. Take, for example, all drug laws. Drugs are criminalized because they harm the individual who takes them, and because we want to decrease exposure to them. In that case, even though legalization would probably decrease the violence associated with the drug trade, we justify its illegality for other purposes. Maybe that's a good thing, maybe it's a bad thing but the fact is that all criminal law is created because we want to discourage behaviours that are deemed bad for society, and "bad" isn't limited to violence, nor does it need to be. Nor, in fact does it include all violence. Just as we criminalize some behaviours that aren't violent, we expressly allow others that are - such as: use of force by police officers, or contact sports. Law doesn't have to be just about preventing or reacting to violence. We criminalize murder for the same reason that we universalize healthcare or make any other law - because it is how we want our society to be defined.
My point is that there is no need to find some causative link between sex crime and rape pornography before it magically becomes OK to make it illegal. It's about culture building, and no culture isn't causative of behaviour but it defines the context in which we can choose how to behave. There have been entire societies where pederasty was particularly normal and yet today, it is almost universally regarded as repugnant. Can we actually say that the culture in which we exist doesn't influence those views? Can we say that their culture didn't promote it or have a "causative" effect? I'm not denying free will; I am merely saying that our culture and our circumstances can put before us choices we would never have to make - inspires desires that we never would have had - if we lived in a culture that was different; if instead of being rich we were poor; if instead of having parents we were orphans. Culture can define the choices that we make, not by forcing us down a path, but by selecting which choices are put before us.
And ultimately, the law is about culture and society shaping. And the question is: do we want our culture to be one which encourages, enables, or tolerates the viewing of rape as a source of pleasure? There are few crimes as repugnant, and we can't keep pretending like our culture is blameless when it happens.
As for freedom of expression, it exists to protect values; it is not a value in and of itself. We protect speech that promotes political self-determination - to allow others to express views with regard to how we want to govern ourselves. Or maybe we will protect speech for religious purposes, or maybe sometimes even for commercial free market values. The point is, freedom of expression matters for the values it protects, and not on its own merit. A murder could easily be a form of political expression - we prohibit it because we value sanctity of life more. In that sense, expression can and should be restricted when permitting it would conflict with other values that our society holds more dear.
What value is that we protect when we permit rape pornography to exist? And does it outweigh respect for human dignity? respect for sexual autonomy? respect for women? Respect, indeed, for the whole human enterprise and the process by which life is created.
It's not some slippery slope. It's not about "anything that could be considered offensive." It's about finding a balance between the values that freedom of expression is meant to protect and the other values our culture wants to promote.
To my mind, respect for women, human dignity, and sexual autonomy far outweigh any value that could possibly be promoted by rape pornography (which I am at a total loss to even imagine - privacy perhaps? sexual gratification? As I've said earlier, I can see no value or benefit in encouraging or enabling other human beings to view women [or men] as mere objects for sexual gratification with no sexual autonomy of their own)