There's a great Neil Gaiman blog post about this kind of thing:
http://journal.neilgaiman.com/2008/12/why-defend-freedom-of-icky-speech.html
Fundamentally, the question is whether we prohibit behavior because the behavior itself is harmful, or because the behavior is disturbing. Masturbating to a rape fantasy isn't in and of itself harmful (and no study I've ever seen, nor I believe has ever been done, has shown a causal relationship between extreme pornography and violent acts). I have all the sympathy in the world for rape victims, but in a free society we can't ban something just because we find it disturbing.
If the issue is that a rape victim would take offense at the game, I promise you there exist other forms of media wherein rape is depicted in the same way: as a target of sexual fantasy. You're conflating a game depicting rape with the specific violation of an actual woman, and that's not a fair assessment. You wrote that gamers are "jerking off to the act that scarred [the victim]" but that's patently untrue. They're not jerking off to anything which harmed her, they're jerking off to a simulation depicting a non-existent woman being hurt. Do you believe this game will encourage rape? If you do, I'd ask you to provide actual evidence that it increases the rate of rape. Do you believe this game would some how be played on a computer wherein a woman who was raped would be forced to watch? If so... Wow, just wow. Otherwise all we're talking about is:
Do we ban something because the mere existence of it is distasteful? You equate it to a game being about violence directed at a specific group, but those games exist. Even ignoring Custer's Revenge, you have Grand Theft Auto wherein you kill hookers, you have any number of FPSes in which you kill people or aliens. Boondock Saints shows two brothers killing a bunch of mobsters, did that encourage copycats? Hell, many movies even depict rape, does that somehow traumatize victims? If it does, how does that become the problem of the writer, producer, or distributor?
The irony of this issue (in my mind) is that most people would never have heard of this game were it not for whinging, shrill, drum-beating of those who wanted this game banned. I'd wager that prior to this kind of media attention, hardly anyone (much less many rape victims) would have heard of this. If exposure to the existence of this game is the inherent harm, you've done more to traumatize rape victims than the publishers have (since the game can't now reach a large audience).
If you do a quick google search, I'll bet you dimes to dollars that you can find a huge amount of simulated rape. How does the predilection of one person affect anyone else unless someone is actively being harmed? I don't know any rape victims at the moment (that I know of), but if you want to find me one I'll be happy to explain that the principles of free speech can't be trumped merely by someone's feelings being hurt unless it causes actual, substantive, harm.