Actually, what we call 'Mature' games are not (necessarily) 'mature' at all. In fact, it's quite the opposite--when they say 'Rated M for Mature', they are literally saying that you need to be mature in order to play it. It doesn't mean that the game is mature at all.
After all, in real life if you went around swearing your guts out and shooting things, would you be called mature?