"It is true that the tendency of dreams and of unconscious phantasies to employ sexual symbols bisexually betrays an archaic characteristic; for in childhood the distinction between the genitals of the two sexes is unknown and the same kind of genitals are attributed to both of them.  

But it is possible, too, to be misled into wrongly supposing that a sexual symbol is bisexual, if one forgets that in some dreams there is a general inversion of sex, so that what is male is represented as female and vice versa.  Dreams of this kind may, for instance, express a woman's wish to be a man." 
 

-- Sigmund Freud, Interpertation of Dreams