Okay, so imagine a film with no violence, no gore, no profanity (other than, maybe, "darn", or something mild like that), and no sex. The only thing this film would have is casual, non-sexual nudity. So, what rating would a film like this get if it was a real film? I'll show you what rating I think it should deserve.
or 
at most.
What does "casual, non-sexual nudity" mean? There are contexts in which it is allowable and others in which it is not.
Same as if the people were fully dressed. Just without clothes doing normal things.
In my opinion, I think the MPAA would give such a movie a "R" rating.
Probably. The MPAA just can't separate nudity from sex, even with non-sexual nudity. To them (and most people in general), non-sexual nudity doesn't exist. If it's seen, it must be sexual. It just has to be. Even though it doesn't have to be. That's why they would give such a harsh rating to a film like the one I described.
It would be a R