1. Nature is depicted as the artist painting, or creating, the young man's face. The point being made is that the face is as beautiful as that of a woman, but better in that it has none of the defects associated with female beauty. Also implied is that the face is natural, not disfigured by cosmetics, giving it superiority over a female face, which was so often false and artificial.
2. the master mistress - master implies the thought of masculinity, whereas mistress implies feminity. It conveys the thought here that he is talking about a man, perhaps wishing that this man was a woman for it would then be more acceptable for his passion to be devoted to this particular person.
8. The attention of all men are on this person. They are attracted to him, and don't look at sexuality, rather the person himself. Women are amazed by the sould of this person. They feel a deep connection with him.
11-12 By that one simple mistake that Nature made, it prevented us from being together. The one simple mistake here can be referred to this person's gender, but more importantly it refers to the "one thing" which some may feel makes reference to the anatomical difference between a man and a woman.