It's true that this isn't news. Rape simulators and various types of games/anime are not new. I've seen a few in my day and though I haven't played/seen this one I'd bet it's not the worst of the bunch, either. I think this can be summed up, though, by taking a look at A Clockwork Orange.
In the novel, the main character rapes a couple of ten year olds. In the movie adaptation, however, it's changed to consensual sex with older teens. This, I think, defines the difference. With a book, simple prose, it's just words. It's far less jarring because it really is "just a story." But when adapted to film, now you're dealing with something far more vivid and even in a movie as fucked up as A Clockwork Orange, not many people would be comfortable depicting a child rape scene. Especially since these girls would be played by actresses who are actually real people. I'm all for creative freedom but creative freedom comes paired with creative responsibility. And filming the scene as it was written would have been creatively irresponsible.
I'm convinced the only reason games/hentai like this can 'get away with it' is because it is "just a cartoon." It might be in bad taste but it has the same kind of defenses that the Clockwork novel would use.
I'll be perfectly honest. I think Manhunt pushes into creative irresponsibility, too. But, again, it's "just a cartoon...sort of." I guarantee that if CGI were capable of mimicking reality flawlessly then a game like Manhunt wouldn't be able find a market. Who'd want to play it if it were that real, right? I think the same thing goes for these rape simulators or hentai where the oldest female involved is a whopping 12 years old. If it got "too real" it ends up crossing that line into "what the fuck this is sick" to even the most dedicated supporters.
I'm sort of talking out of both ends of my mouth here. But what I'm trying to get at is that at some point an offensive concept is less offensive to the masses because of the limitations of the medium itself. When moved into a different medium, or when the limitations of a medium are removed from the equation, then all of a sudden something relatively innocent turns into something incredibly disturbing.