You see when I presented this problem to a friend of mine
he suggested that the knot would be solved if we assumed that
Prospero was homosexsual. This is however not a possible solution I think since
Prospero both has a daughter and that his words about his possibily diseased wife are longing:
MIRANDA Sir, are not you my father?
PROSPERO Thy mother was a piece of virtue, and
She said thou wast my daughter; and thy father
Was
Duke of
Milan ; and thou his only heir
And princess no worse issued.
As I read it
Prospero is here not speaking of something which displeased him. If he had been homosexsual he would probably not have spoken in a longing tone when mentioning a past wife. Furthermore he was as duke of
Milan , which should be a powerful enough position to
avoid marriage and keep a male lover instead. If that was his wish I mean.