N0m: What’s your comment have to do with my point or address it in any way? There will be good and not good therapists regardless if they service both genders or only one gender. But if they service only their own gender, they’ll have less opportunity to sin or have Yichud.
Your example of the male therapist specializing in troubled girls makes my case. If the community wouldn’t tolerate opposite gender therapists, he would have been denied the ability to have inappropriately acted as you described.