At The Same Time, theoretically, it should be possible to become so innocent, by the Spirit, that you can behold nudity and not lust. However, if you were to ever reach this point, it's /incredibly/ doubtful that you'd find yourself sitting around observing nude pictures for entertainment.
So, this brings us back to the question, is it okay to look at images of fake nudity? Here I believe I'd find the difference between "looking at" and "seeing." If you're walking down the street and someone flashes hentai in your face, I don't think you've suddenly been forced to sin. Rather, you've "seen" fake nudity, and have chosen to move on from it. This is different from "looking at," where you do not move on, but rather, with intention, take in the image. I'm quite confident it's that "taking in" that is inappropriate before God.
Now, going back to Matthew 5, and taking /that/ all the way back to Exodus 20, seeing as my friend above pointed out its origins in the 7th commandment, you can't just take /part/ of the Law, you gotta take it as a whole. The Mosaic Law, given by God, dictates that either you're committing adultery /or/ you marry the person that you have sex with (or, technically, you get stoned, but let's avoid that one for the sake of this discussion). So, if you want to read the Law into what Jesus is saying in Matthew 5, rather than understand that Jesus is drawing from it to ordain a greater commandment, then we should learn from Matthew 5 that married people cannot lust after people, and unmarried people should get married to the first person that they inwardly lust after. Personally, I find it agrees with the totality of Scripture far better to understand that Jesus was explaining a greater commandment, rather than attempt to read Jesus' words through the lens of Mt. Sinai.
Where does this leave us? Can you marry the image of fake nudity? No. Therein, it does quite seem that you are flat out in sin for desiring after it in a sexual manner.
For your brain, there's no difference between the fake image and a real image, even barely a difference between an image and a person, so neurologically you actually have bigger problems (it's flat out the same thing), but I've been trying to stayPost too long. Click here to view the full text.