I don't think there is a 100% clear line- unless it's something like a fapfic glorifying underage rape/ non-con, sex etc. Then it is a very clear, hard hell fuck no kill it with fire! But for non-con, or graphic punishment- I think the line is blurred more, and the answer to the author's intentions can be found in their works- the quality of the writing and how are they handling the subject material. Not that high quality writing is carte blanche to write however/ whatever.
I've dealt with heavy subjects in some stories- and some of those scenes were quite graphic. Like in Bad Seed, severe child abuse but the focus was on the emotional and physical scars it left on Cameron and that pain affected him past and present, and his journey in coping and moving on/ learning to deal with it. Part of that included a graphic flashback to an abuse scene, but from a story standpoint that scene was important because it was so life-defining for him and other members of his family. It was a catalyst and I felt the audience needed to emotionally experience that to better understand and identify with Cameron's struggles.
but getting back closer to punishment porn tropes, in general (painting with a broad brush here) and I know this has been brought up before, but audiences seem more tolerant/ accepting of a mother or female authority figure abusing a male character than they do a male authority figure abusing a female one. I know some stories featuring the latter have received a lot of backlash while the former get praise.