Supposed Reasonablensss
Conceptions of normalcy are mostly decided by professional consensus. There is no such thing as a value-free description of normal behavior.
I am a relativist, a viewpoint which is pretty common in my field. I don’t see why the views of normalcy held by professionals, and I include those in my own field (sociology), should be privileged over those held by anyone else. Psychotherapeutic concepts of what is normal are value judgements based on research or clinical evidence.
IMO, reasonableness is just a convenient fiction. In order to have reason, one must begin with logical premises. It is in those premises where bias begins.