I think the main reason why nudity seems to be so much more restricted in videogames than violence is, is because its damn near impossible to may nudity "comic" so to speak.
The fact is, violence often requires detail to appear in any way similar to the real thing - gore, blood splatter, bone fragments etc. - and it's only recently that computers have had the power and capability to represent such details. Still, its not 100% realistic, you would need huge amounts of computing power to accurately represent someone's head being hit by a half inch bullet for instance - just the physics for animating brain fragments would be rediculous. This has all stemmed from our origional toleration of violence when videogames were just blocks on a screen and something dieing was "Pew, pew, pew! Urk!"and maybe some pixels fell over which was seen as something that simply wouldn't affect someone's mental development due to how distant it was from real death and violence. How "comic" it was, to return to my origional point.
Nudity however, was considerably harder thing to make "comic". Even with a block of pixels, it was possibly to represent all the approprite parts of the anatomy and, at least to a point, mimic real nudity to where it couldn't be explained away or considered to distant from reality to make an impact. Take the previous example, the dead pixel creature - there are a million other far more innocent alternatives to "kill" such as "stun", or "KO" or "Sent to sleep" which could sensibly be substituted for "kill" - in the end however, its just a way of removing something from the game, or at least, to prevent it troubling you. Nudity, especially the sexual side, cannot be explained away legitimately like this, or distanced from the reality. For violence, if you punch someone, they fly across the room, bounce off a wall like rubber and land without a scratch on them, any well educated person, even a child, would laugh because that's so far detached from the reality. They will understand the sillyness of what they have just seen, dismiss it, and keep a clear barrier between the fictional and the real - most will do this even with considerably more realistic violence (well perhaps not laugh, but maintain the boundary) because they know the reality.
Nudity is a different matter - how do you make that "comic"? Even "comic" depictions of people usually bear clear similarities to real people - and real nudity - partially because nudity is a passive act - "being nude" - rather than violence - "to commit violence" - which is active and so can easily be made comic purely by exaggerating that action. Nudity doesn't work like that (try exaggerating nudity and things, instead of becoming comic, become sexual, most of the time) and once you move toward the things that are often associated with nudity - read: sexuality (even when its not, its usually assumed) - it becomes very shakey ground. We can trivialise violence - its called slapstick and it's funny if done correctly - but trivialising sex is another matter. For one, how? It's still sexual no matter how you portray it and two, it still has the full impact no matter how you explain it (unlike our "kill" and the less violent "stun" example). It's hard to differentiate "comic" sexuality from reality purely due to its nature, it is hard to have varying degrees and objectives (so to speak) regarding sexuality unlike it is with violence - the word says it all, sexuality, having to do with sex (usually in its more colloquial interpretation).
End point is, nudity is hard to make "comic" and so differentiate it from reality and demonstate the line beween the real and the fictional, especially without making said nudity appear sexual. As for game content which is sexual, well, would you want sexuality trivialised? I think the main point is that videogame censorship seems to have been running on the same ideas since games first came into existance, and has failed to adapt as they change, leading to such high levels of realistic violence being accepted as normal in videogames, and, to an extent, perfectly harmless on the psyche. Nudity has never been seen this way due to how well it can be represented even in a minimalist fashion, and so suffered full blown censorship from the start, especially due to the way it could be portrayed, and how it could never be "comic" in the same way violence was.
About your art argument - yes, the nudity in art is accepted but primarly because art is viewed as something quite different to videogames. Sculptures are viewed as tributes to the human body etc. etc. (this is actually getting a bit Nazist...) while videaogames are more seen as trivialisations of real occurances, and trivialisation of the human body itself, and sexuality, could easily be seen as capitalist exploitation, or such things. Its all been caused because the world of videogaming has blundered forward with all the associated authorities failing to realise just how it works and always atttempting to nail the tail end to the floor without thinking what they're really doing.
I hope that made sense. If it didn't, say so and I'll clarify.
EDIT: Who is the Queen of Thorns btw? I must have been asleep for that...and holy - how much did I write?
