However, I am in full agreement that videogames that just use sex and stuff for no other reason then to say 'WE HAVE A SEX SCENE IN OUR VIDEOGAME!!!' shouldn't be allowed in, mainly because they approach mature issues in immature ways (which doesn't do anyone an ounce of good). If, on the other hand, they approach the issues of violence, sex, torture, terrorism etc in a way that gives clear indication to its seriousness, then by all means, allow an R18+ rating for those games (if it is extreme enough for it). For example, MW2 has a terrorism scene where you actually kill countless civilians, but not only does it give you the option of skipping it (and warning of its maturity in the process), but it also portrays it as something wrong (with the whole 'I'm a bad guy trying to start a war against America, and that is why I am killing these people MWHAHAHAHAHAHAHA' thing).
So, yes, we should have an R18+ rating for video games that approach mature issues in a mature way, but we should also not let any games which approach mature issues in deliberately obscene and immature ways get a rating.