It seems like you are defining beauty AS usability. To me they are orthogonal. Beauty is a quality that appeals to my aesthetic sense, and usability is a quality that appeals to my practicality. There are beautiful websites that are unusable, and ugly websites that are very usable, I'm sure you can think of examples for both cases.
I believe perpendicular applies to only two dimensions, while orthogonal applies to three or more. The latter makes sense when there might be more than two dimensions present in the discussion.
