Jewish Courage,
I would think that a phycologist in many ways should be compared to a teacher.
It is prohibited for an unmarried man to teach children because of the fear he may develop a relationship with the mothers who bring them to school (Even HaEzer Siman Chuf Beis) while there is some disscussion about when this halacha is applicable nowadays where we have school buses, I would think it would be quite logucal to infer that it is of sourse prohibited for an unmarried man to expressly treat an adult married woman especially since treatment usually nessacitates building a relationship.