In Jungian theory, as I understand it, the mother-son complex can (unsurprisingly) result in the extremes of homosexuality, "Don Juanism," or impotence. In the first of these, the son's "heterosexuality" is unconsciously restricted to his mother. This would seemingly explain why he doesn't seek actual relationships with women but instead is gay. In "Don Juanism," the opposite result occurs: in each new sexual conquest, a man sees the image of his mother, or he is endlessly seeking his mother in other women. These concepts need to be understood alongside Jung's notion of the anima as a feminine element that exists within the personality of all men. The strong attachment to the mother "archetype" can bring out the anima characteristics in a man, but not necessarily to the extent of destroying his basically masculine nature. Perhaps in Psycho's Norman Bates, we see that the anima does actually obliterate the rest of his personality.
While all of this may seem extremely artificial and schematic—especially in our time, when sexuality and sexual preferences are not judged and analyzed critically or clinically in this manner—it's not hard to see the application of it to Norman Bates. In Norman's mother-complex, the mother "archetype," as Jung defines it, has taken over Norman so completely that he is a multiple personality who becomes his mother, putting on a wig and dress and then killing people. According to the convoluted explanation the psychiatrist gives at the close of the film, Norman is not an ordinary "transvestite," or cross-dresser, because he is not wearing women's clothes to accomplish a "sexual change" but rather in order literally to become his mother. But in Jungian terms, this may simply mean that the mother's influence upon him has been so overwhelming that the anima has taken him over, at least during those phases when he becomes the mother.
But what of the other personality, the "actual" Norman Bates, dressed as himself and showing a façade of normalcy? The Jungian explanation would probably be that Norman's mother-complex has resulted in his being dysfunctional with women and impotent. His attempts to flirt with Marian Crane obviously go nowhere, and then he murders her. One might more conventionally see this as the result of a psychotic rage in which Norman blames the woman for his own impotence. In my view, the film's psychiatrist, in explaining Norman's multiple-personality syndrome, is giving us a kind of pseudo-psychological babble unsurprising in a time (1960) when Hollywood was first beginning to open up and deal in a sensational manner with issues of gender and sexuality. But as stated above, Norman's behavior can indeed be seen in Jungian terms as the mother-complex in which the son's personality is completely overwhelmed by the mother "archetype" and the anima becomes dominant to a psychotic degree.