Additionally to what RockyRaccoon said: it doesn't.
There is no rape in Grand Theft Auto, there's not even sex on-screen (occurrences are only referenced in dialogue - where it is, to my knowledge, always consensual).
And again, there is just no sound evidence that violence in fiction (movies, games, etc) causes a rise in violence in real life... here's a graph that shows the number of crime victims per 1000 US citizens together with the release of some particularly violent games:
(note again that correlation doesn't mean causation; the fact that the rate is sinking isn't caused by the release of the games, it's simply a coincidence)