I want to fret one string and use the adjacent string as an open string, but there is no way on earth that my finger can NOT touch the open string. I think I'm a pretty normal adult.
I've seen people discuss this from every point of view and it is always (that I can find) about fretting one string nicely (trimming nails, posture, position, etc.) and never about not touching the adjacent string which needs to be open. I don't have a problem fretting, or intentionally muting the adjacent string. The problem is that I always touch the adjacent open string. Just take any finger and fret any string (pressing straight down) and tell you can do it without touching the adjacent string.
I'm using a classical, which has a wider neck. I have not had it setup, but it seems pretty good in that respect. It's not expensive but the neck is straight, the frets are flat and even. What else can I look at? I'm not taking lessons due to Covid and an answer of "lessons will sort this out" is fine.
(I'm new here and any comments on style are welcome.)
EDIT: It is the answer. All other answers I have seen just say "keep practising and you'll get it", but none of them say how you can "get it" when a single finger cannot physically fit on a fret without interfering with an adjacent open string. The linked article talks about that and makes it clear the answer is "keep practising" but also says why.