R18 certificates are not an excuse for being a bad member of society
I've seen many disturbing titles, far worse than Grand Theft Auto. But this was when games were a cult market, and the technology was so basic, it was easy to dismiss the phenomenon.
Now, games have matured. They can do more and they reach more people. Like movies and other entertainment, they have the power to influence society. With this influence comes some level of responsibility. It's not an obligation - games are art and should be able to push boundaries - but rape in a video game is not art. It's gratuitous indulgence in a sickness, a sickness that hides behind freedom of expression.
If we're going to get philosophical about it (and I guarantee the proponents will) you could ask “where does it end? Should we ban all violence in games?"
There wouldn't be many games left. Maybe we should address the high level of violence in gaming. At least it's a conversation we should be able to have without all the "freedom of speech" bullshit. For now, we can at least draw the line at what is acceptable behaviour.
Would we allow child abuse in a video game? The creators would be locked up just for suggesting it and rightly so.
So why do we allow rape? It's even banned in porn, and used sparingly in movies and books when the story requires it. Gaming, putting the player in control, has a different context. Because of this it plays a different role in popular culture and should be assessed differently.
Killing might not be particularly healthy content, but at least it can be justified to a certain extent in games, and is; demons, war, self defence, etc. Many games (Batman: Arkham series, Deus ex) actively avoid it. It's rarely gratuitous - a game where you could shoot up a school should offend even the most hardcore gamers.
What kind of moral integrity do you have if you're actively campaigning to include abuse in entertainment? What does it say about our culture when people lobby in favour of sexual violence?
This isn't about the freedom of adults. It's about the freedom of assholes.
Despite what proponents might say, there is a link between gaming and real life - it's called culture. Entertainment shapes it and it shapes what people find acceptable. Abusing the vulnerable is not something we want in our culture.
Yes, the world is a violent place. Our popular culture doesn't need to hide us from this. But it really doesn't need to encourage it.