I think you Europeans are looking at it from a very European perspective and I honestly think you've got it wrong. Don't mistake my tone, I think you're taking a rational look at the problem, but that isn't going to work here. I think the root of this problem is just the opposite.
Before I continue, lets try not to take this thread too far off topic, lest it be closed. Also, this is merely my opinion based on observation, this is not an attack on anyone and does not require anyone to get defensive or even aggressive in response. I'm simply calling it as I truly believe it to be.
I honestly believe that this moral issue of violence, sex and nudity in the US stems directly from the far right-wing, ultra conservative Christian groups. They have the loudest voice and considerable political muscle, but the narrowest world view. They base their lives off the exact wording in the Bible, taking every word as the direct word of god and therefore, undeniably true. Now that's their right, but when you approach the world from that angle, violence is ok (even encouraged against anyone perceived as not believing the exact same as you), but nudity is wrong and sex is evil - something to be avoided unless for procreation. That's how it is in the Bible (yes, I've read it), so that's how it has to be in the real world. Being a particulary loud minority, those beliefs then filter out across the nation via the media, resulting over time in a country whose people's moral standing on many issues has a lot more in common with those of certain middle-eastern nations than they'd like to believe.
If you think about it though, there's a pattern across the world. The places with the greatest tolerance for violence but the least tolerance for anything remotely sexual tend to be those where the religious majority is more conservative. Countries with more moderate or liberal majorities on the other hand, tend to see sex as a good thing and violence as something to be stamped out or censored.
Considering today's political climate and the strength of certain lobbying groups in the US, I don't think you're going to get the "adult" games you want. Apart from the fact that it reduces sales and thus profits, there's no way to stop some obnoxious, know-it-all brat from getting the games anyway. A couple of clicks on the internet can get you anything and parents know it, but instead of taking responsibility, they try to pass the buck. It's something they've learned from all those greedy lawyers - it's always someone else's fault. They should fix it, not us.