You do not owe explanations for every boundary around information.

A boundary that has to be justified is really a request for permission. If you explain why you will not discuss something, the explanation becomes the thing being argued about, and you end up negotiating over ground you had already decided. The refusal is complete on its own. Reasons invite counterarguments.

Practice a version with no rationale attached, said kindly. Something like preferring to keep that to yourself, then a genuine question about them. Most people follow the redirection without noticing they were turned down.