I think there are good criticisms to be made on an artistic level. There are some really dreadful films that can be similarly criticized.

There is the concern that it will effect behavior. That is one reason why virtual child pornography is made illegal. But this also gets into free speech issues. Some countries outlaw pornography altogether (Iceland is seeking to ban internet pornography in addition to already banned printed materials).

The US is unique in the breadth of its protections of free speech, but an argument can be made that pornography and violence do not really qualify as speech - at least not speech worth protecting. [I don't really see how that can work without harming free speech, but it's not a crazy argument]

I don't really know where to go with it either. I don't think gamers care so much about artistic merit...and in some games the violence may serve a great expressive purpose.

Is acting out violent or sexual scenes worse than watching them in a film? I don't know.