I agree that getting aros to define romantic attraction is inherently problematic. When I'm explaining my aromanticism to people, it's usually a bit of a long conversation, not a succinct definition. I usually say something along the lines of, "I have never 'liked' someone in a romantic way (such as a crush), and I'm not even entirely sure what it means to 'like' someone. I just don't experience feelings toward anyone that could in any way be classified as "romantic." I've never desired any sort of romantic relationship with anyone, because it's impossible for me to see someone as anything other than a friend." I have no idea how to condense this into a definition that would make sense to everyone.
I feel like to get at a decent definition of aromantic, we need to have both aros and romantics involved. Romantics often have a difficult time putting romantic attraction/feelings into words, but at least they know what it is. We would just be guessing.