You are again trying to use and explain the logic linking sexual activities to automatically means not being pure - honestly, you may not realise it, but it very close to the way moralists would talk about it. My perspective is different.
Even sticking to sex, I met many "bad people" (to avoid language I would otherwise use) who did even less "unconventional" and "extreme" things, and people who did that kind of stuff, yet they were naive and pure from a different perspective.
Said even more brutally, you are using a reference what happens between the legs to decide if they are pure, I do not exclude completely that, but I focus first and foremost on other elements, on the "why" as much, and for some thing even more, than the "what", and for me from that point of view Katherine is a child in young woman's body exploring sexuality.
If instead of William and Jenny (whether together or separately), she had met other people, her story could go very differently.
However, I am not trying to change your view, or of the ones that have a view closer to the one you are expressing.
Both of our points of view are legitimate, insofar as they are our points of view as individuals, just don't continue trying to explain me the perspective used by moralists, I know it perfectly from a long time, I just don't adhere to that
.