Unity (as far as I know) doesn’t support genuine quad based meshes. This is presumably for simplicity and probably compatibility (as many simpler GPUs don’t actually support quads natively).
When you perform a SetIndices as in your example, unity uses your request to calculate triangles from your quads.
So short answer, you can’t really. I won’t ask you why as it sounds like a terrible story, but if its a visualisation thing then you could look into a wire frame shader that explicitly hides centre lines of quads through some means.