Hmm... looking at the list for AO games...
The list of AO-rated products is composed of entertainment products that have been rated by the Entertainment Software Rating Board to have strong sexual and/or violent content suitable only for persons aged eighteen or older. The AO (Adults Only) rating is seldom awarded; in the fifteen year history of the organization and thousands of products rated, only twenty-three titles have officially earned and kept the rating. Out of these twenty-three games to be rated AO, twenty earned the rating for just the sexual content, two were just for the violence and one for unsimulated gambling.
Looking at the list, "Strong Sexual Content" means "It's Porn", which probably means it wasn't getting too hurt by having a no-no rating, since my experience with media has generally told me that Porn tends to do very well for itself, thank you very much...
One is actual gambling game (I guess it's an actual CD you use when you gamble online, instead of just a website?), which the law is trying to slam down on with more extreme measures than just restricted ratings.
One is Manhunt 2, which proudly wore the title "Murder Simulator" for as much money as it could make in the aftermath of the GTA controversies. Oh, yeah, and apparently, it has "Strong Sexual Content", too, so apparently, someone slipped a nipple.
And finally, there's
http://en.wikipedia.org/wiki/Fahrenheit_(video_game) Farenheit / Indigo Prophecy, which is the only game on the list I actually played, and I can't really see why THAT of all things has the highest rating. The game is supposed to be more of an "interactive movie" with a heavy slant on plot over what essentially amounts to "bop-it" style gameplay in action sequences and a few decision trees. What, was it the fact that the hero is an atheist who enjoys reading "Thus Spake Zarthustra"? Or was it the whole being raised from the dead by a free-floating AI ghost that turns into a raven and talks to people? Anyway, I'm not sure if it's "art" or just trying really, really hard to BE art, but that one actually does try to kinda-sorta be mature. --- OH WAIT! Apparently, reading down, only the "Director's Cut" had the sex scene, and was AO. So apparently, it doesn't matter how many zombies you behead, so long as you don't have an interactive sex scene or actual money gambling, you're pretty much safe from the AO rating.
Now then, on the "Temporary AO" list, we have GTA, because of that silly "Hot Coffee" thing, because, of course, you can blow up as many Los Angeles mock-ups and shiv as many old ladies for no real reason other than to hear them scream you want, it's still safe for kids until you have a minor sex side-game.
The rest of the temporary AO stuff is generally nudity, as well.
So basically, AO means "You can see boobies and weewees here!", which generally doesn't do much to really hurt most of these games, if they're seriously going for sex selling, anyway.