I think people have been saying this already, but sex/violence/drugs in a story do not make it "mature". Instead, they can limit your audience, and why would you want to do that? What makes a story mature is the viewpoint from which it apporaches these and other topics. Books like "The Maltese Falcon" have sex and violence in them, as well as heavy drinking, but the way they're written in doesn't emphasize them. I think it's a well known fact that people drink, have sex, take drugs, and kill, but the more hollywood-style approach you take, the more inaccurate and cheesy your plug-in will be. Over-the-top characters just aren't...as good.
Take the movie "13 conversations about one thing", for instance. The story doesn't allow you to totally hate even the more villianous characters, because of the depth and dimension given to them. Everybody is ultimately human , with both good and bad points of their own. It deals with things like crime on a level thats not so gritty as it is "heavy"--that is, it's hard to come away from it without any emotional impact whatsoever, and it has a wonderful mastery of subtlety I guess you'd have to see it to know what I mean, I don't think I'm articulating it all that well.
For romance, the more "maid in manhattan" you get, the more I'm likely to lose interest (and I'm sure I'm not the only one). And sex, well...if people want porn, they'll go look for porn. There are few "adult" things LESS mature than pornography, and the less cheesy, graphic, and pointless sex I see in games, the better.
Lastly, I think it's nonsense to think that, realistically, drugs and illegal substances aren't around in the future. Drug-tinged elements in your plot wouldn't be very far-fetched (depending on what you write, of course). Again, just try not to make it the theme or focus of the game. And if you haven't had much experience with these things, at least do some research first, else you risk unintentional "blooper" humor in parts that may be intended as more "heavy-handed". Just like how you want to eliminate a many typos and grammar errors as you can: You don't want to come off as a fool.
...Geeze, I hope that's all intelligible. Went for a bit of a ramble-trip there.
{edit: speaking of typos...}
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(This message has been edited by Onyx (edited 04-21-2003).)