I seriously doubt that she considered your holding the door open an assult. She probably considered it disrespectful, and unless you are equally likely to hold a door for any person, then it could certainly be considered condescending from a certain point of view.
The issue isn't that intent doesn't matter, it is that intent doesn't matter more than the subjective experience of the second party. Their perception of your actions is equally important to your intent. When you are able to accept other people as equals, and not as subordinates, you'll understand why that's the right way to handle the situation.
Again, that doesn't mean that a complaint is always valid. Mediation is intended to determine both intent and perception, to explain both to each party, and whenever possible, to provide guidelines for everyone to ensure that future interactions don't create further offense.