It's crap like this that makes me glad the constitution guarantees the right to face your accuser. If only that applied to personal disputes and office politics. I've always felt that if someone has a problem with me they should either deal with me personally or just shut up and move on with their life.

My advice is simply to stay calm. Getting upset tends to lead to bad decisions and will not help your case. If you remain composed and dignified yet firm and resolute, it will contrast well with your accuser's obviously irrational state of mind.

A few other thoughts:

I think she has point; She should be able to wear jeans. Really, why not? However the argument does not work in reverse. If their is a policy preventing her from wearing jeans, it does not follow that you should be robbed of your right to wear a kilt.

I do think that it could be that they simply do not see the difference between a kilt and a skirt. If you have worn skirts and kilts, it can blur the line in the minds of many people.

I think their concern over your job performance is entirely understandable. If your kilt is offensive to some people (irrational as that may be) it could potentially cause problems. The fact that you didn't know that your coworker was bothered (and that you still don't know who it is) may also hurt your case, as it shows you may not always be able to tell who will be bothered and who won't. I'm not saying they are right, only that their arguments are not completely without merit and should be carefully considered before you respond.

I had a situation with a friend who could not handle my kilt, and called it "cross-dressing with an excuse" and it nearly ruined our friendship. By trying to remain understanding and forgiving (while not letting her dictate my clothing choices) I managed to salvage our relationship, and she is getting used to the kilt bit by bit. The key was understanding that she had a problem, and that this wasn't simply malice on her part. This may not be applicable in your case (especially since you don't know who it is) I don't have enough information to say what's going on in her head, but it is a good idea to keep in mind that she probably doesn't choose to be ignorant and irrational.