I think that the main thing that keeps nudity/sex and violence separate is a simple one; violence is always considered wrong in real life--sex/nudity is not.

It's so much easier to explain this to a kid. When he's killing hoards of enemies in a video game, it's very easy to explain and for him to understand that this is fake and it is never an okay thing to do in real life. Witnessing a sex scene, on the other hand is different as you can't explain to the kid "this is horrible and you should never ever do it." It's more delicate. It's more nuanced. As such, it requires a higher level of maturity for the kid to be able to understand and comprehend the proper dealings with it.

That's how I feel, at least.