[Nymph Philosophy] Why the nude female form?

[davidjackson]

Firstly, I think at historians prefer the term nude to naked in this case. Naked is often associated with the forced lack of clothing, with the shame and taboo that comes with such a situation. 
Nude lacks such connotations, and this is certainly the case in the manuscript. Everyone seems as happy as a anyone in an fkk camp! 
Secondly, there is a case to be made for the sexual overtones inherent in these forms. Long hair and swelling bellies, together with smooth white skin as depicted by the ability to blush (an indication of the person not having to carry out manual labour) were typical inductors of femininity in the middle ages, as we can see from sculpture and paintings of the time.
That is not to say that I detect any erotica in the book. Rather, the nymphs appear to be self confident depictions of assertive women. 
They most likely are, as jkp said, depicted animii.