It is forbidden to be secluded with another woman (excluding a woman that it is permitted e.g. wife, mother, grandmother etc.). This is known as yichud.
It is permitted though, provided the area is not fully secluded and e.g. the door is open / door not locked, people can come and go, and there is potential for someone to enter. Or if the woman's husband is in town.
However these allowances only apply when the relationship with the woman is not Libo Gas Bah, meaning the relationship between the man and the woman is not comfortable and intimate. See Shulchan Aruch Even HaEzer (כב-ח):
אשה שבעלה בעיר אין חוששין להתייחד עמה מפני שאימת בעלה עליה ואם היה זה גס בה כגון שגדלה עמו או שהיא קרובתו או אם קינא לה בעלה עם זה לא יתייחד עמה אע״פ שבעלה בעיר:
A woman whose husband is in town - there is no concern for yichud. However, if she is "Gas Bah", i.e. you grew up with her, or she is a relative... it is forbidden to be secluded with her even if her husband is in town.
Therefore, is a male allowed to have a female therapist? Assuming the office is frequented by many people, and the door is not locked. Is the relationship between patient and therapist intimate? The reason to assume so is that the set up of the relationship between therapist and patient is one of having intimate conversations, where private details are told over and nothing is considered too taboo. Is this included within the definition of Libo Gas Bah?
Assuming it is, does it matter that the intimate conversations are one sided from patient to therapist, is that considered the definition of Libo Gas Bah, or it's only considered such when it is a mutual give and take between man and woman? Does the fact that the relationship which, although deals with the personal aspects of the patient, is really in essence a business relationship?