"Graphic design is purely in the realm of the aesthetic."
This is not strictly true. For example, when designing for print, a good designer pays strict attention to information architecture and type readability and legibility (which are two different thing). These things are both form and function.
But then, as I think Donald Norman says, "Beautiful things work better..."
I would say, UI design != Graphic Design. UI design is inseparable from UI function. They are one and the same. Design isn't how something looks.
It shouldn't be confused with graphic design though, which is really how something looks. Graphic design is purely in the realm of the aesthetic.