I think the therapist is right - he does not need to explain why something makes him uncomfortable. No one needs to explain to another why they do or do not like something. A client need not, if they do not want, explain to a therapist either. In the therapy situation, the therapist explaining is making it about him and his reasons - there is no, to my mind, benefit to a client knowing whatever weirdo reasons the therapist might have. The reason I do not see the refusal to lead to mind reading is that the therapist has clearly said - no I won't do X because it makes me uncomfortable. One is not guessing. The reason the therapist is uncomfortable is irrelevant. The point of boundaries is not that someone will never bump into them, it is backing off when one does - accepting it is there and going on. The therapist is not withholding insight about the client and the therapist's reasons would not lead to insight on the part of the client. His discomfort is about him - not the client. The insight for the client, as I see it, is why, when someone tells them no that makes me uncomfortable so I am not going to do it, do they keep pressing the issue? In some ways demanding there be a justification of the stated discomfort.