The one I see says people come to therapy stoned or otherwise all the time. She made it sound like no big deal to her.
I would not continue to see one who acted all upset at me about anything I did. They do not get to lecture me. Not their place to do so in my opinion. Also, I interviewed over 30 of them in the last 4 years. I have not had any of them show me any clause about coming in impaired (which is not a deal for me because I do not, as a general rule, go anywhere near a therapist not in full control of my faculties) - but as a matter of principle - I would have left any of them who told me such a thing. I also only see those who work alone, so maybe that has something to do with it too.