homosexual fantasies

A haredi social worker brought to supervision a case of a 19 year old haredi boy who mentioned that he is physically and sexully attracted to one of the students in his yeshiva and that he has had been sexually involved with two boys the last seveal years (excluding sodomy). He acknowledged that he is also attracted to women and frequently masturbates while fantasizing about sexual involvement with his male friends. After several meetings he expressed interest to overcome his attraction to males and accepted the therapist's injunction to cease contact with his two homosexual friends. However, he was not able to discontinue masturbating at this time. The supervisor suggested to the therapist that he should request the patient to restrict his fantasies while masturbating to females. The therapist raised the question whether this was halachically permissible.