No, not THAT kind of "adult" games.
I am an adult. I want to play a game that treats me like one. I want to be able to approach things seriously and intellectually. I want to not have to wade through puns, pop-culture references, and wacky madcap slapstick humor. I want to play in a RPG session where nobody laughs.
A good game (or a good game master), doesn't treat the rules like they exist behind some ineffable curtain that I'm not allowed to know about. A good game (master) trusts that I can play 'responsibly' and 'intelligently' now, and lets me play the invincible flying sword princess or whatever right NOW, instead of having to slog through six months of play. A good game (master) trusts that I am capable of making my own choices, and standing or failing on my own merits.
A good game doesn't collapse into side jokes, kibitzing, or anecdotes. This may seem unusual, since it's something we're all used to. I do it too. But consider: if the players are cracking jokes, they aren't paying attention to the game, which means that they are not interested in what's going on. Player jokes are a signal to the GM that something is horribly, horribly, wrong. The players aren't interested in each other's scenes, they aren't interested in their own scene, or the "action" scene is bogging down in resolution.
But I'm with my friends you say! It's only natural that I would want to do that sort of thing.
Very true. But four compare hours of joking around with game books on the table and some haphazard roleplaying to two hours of rock solid roleplaying with an hour of solid hanging-out on either side, and I think the choice is clear.