American culture is extremely bizzare. I've never understood it and probably never will. For movies released in theatres or video games the rules are basically: Ultra sick violence=A OK. Nudity or sex= OMG CALL THE FBI!!! THIS PRODUCT IS UNSUITABLE FOR HUMANS!!!
It doesn't make sense if you over simplify it. Like saying that its ok to chop off a breast but you are not allowed to kiss it.
However there is more to it. Every kid can see that violence is wrong. If someone is shot every kid knows that something bad happened. Our internal violence filter is pretty robust in modern worlds.
Show them a Bud Spencer movie and they know its just fun. Show them a violent movie (e.g. a war movie. SPR, EatG, ABtf) and they instantly know that violence is bad.
But if kids see people having sex they might try it too (looks like fun, doesn't it?

) and being kids they don't know much about what happens afterwards and those after effects are rarely thematized in movies. Pregnancy, infections, HIV tests etc.
So in a way the American rating system isn't completely off the hook and for absolut prudes but in a way it makes sense. Its over-the-top in a lot of cases but the core principle isn't as bad as some people make it seem.
Anyway, someone mentioned that Fallout 1 and 2 were "only" rated M and AO, but please take into account that back then videogames had a lot more freedom in that regard and Jack Thompson was still playing GTA 1 were he was "wasted" all the time so he developed his GTA-hatred.
Also Fallout 1 and 2's graphics left a lot (almost everything) to the imagination and anything regarding sex, drugs and other fun stuff ony happened in conversations or other texts.
And text can't be bad anyway. Ever seen a rating for a book? I haven't. Kids are not allowed to play Rainbow Six but they are allowed to read the novel (which is gorier and has an ethically questionable ending).