"Donnie is a young man who thinks constantly in girls and experiments sexual pulsions. About that matter, his parents (specially Rose) are naïve at best. Her mother is afraid and prefers Donnie attending therapy more than confronting sexual growth in a chat. She pays another woman (Dr. Thurman) instead. The therapist is friendly and Donnie reveals her some of his fantasies. It's possible that Donnie is feeling some incestuous impulses to his sister, since he hasn't had real experience with girls.
In the Director's cut, Elizabeth asks him about Gretchen while carving a pumpkin and is curious in the Halloween party when Donnie goes upstairs with Gretchen. This may suggest Elizabeth takes some kinda control over Donnie's sexuality in the same way the matriarch of the family does. Donnie endures all this family control only in his subconscious, but he isn't capable of admitting it. Uniquely in the first dinner scene he gets rebellious remarking disdainful comments to his sisters and insulting his mother afterwards. His search of sexual realization is giddy, leading him to a classical teenager schizo process that forms in his mind the culmination of a monster shaped bunny: Frank.
The bunny is the real Elizabeth's boyfriend, equalling him in his psyque as a sexual rival. However, the external form is devoid of human attachments, only a grotesque suit, masking his guilt. His virtual TV fantasies and matriarchal repression, all this fight is reflected by the artificial evil bunny, who scares him dreadfully in his visions non-stop. Franks is by this logic Donnie’s psyche reflector. This also would explain his sudden way of asking Gretchen to going out with him, 'cause his desire is so intense.
Evidently Gretchen is the perfect girl, because she comes from a disfunctional home and hides herself from a violent male patriarchal figure. She is an angel to Donnie's eyes in cause of her romantic behaviour but most definitively by her sexual freedom, which separates her from the other scholar girls Donnie has met. Gretchen stops Donnie's advances in a scene showing him that an emotional contact must be involved in their future sexual relationship too. She's wiser in this aspect...
So summing up: the female figure is Gretchen and the male figure is Donnie. The smurfette lecture reveals us how important sex is for Donnie: for him life has NO MEANING without sex. When Donnie and Gretchen culminate their union, they are happy and isolated from society, so society is now ready for punishing them. Donnie’s hysteria makes him to run away with Gretchen, but the tragedy will appear when Frank -the evil reflector of American matriarchy and its inherent obscenity (repression)- ends killing her. Donnie prefers awakening dead than suffering a surrender to the dominant point of view of the society. "
(in request of "Search for the perfect movie" by Cinephilia)
The preface can be read here