When in Rome... or in other words, if neutral ground, then wear what you want. If they don't like it, they can leave. If someone (other than spouse, even sometimes then) makes a promise that you will do something without checking with you first, then they must learn that they don't have a say, and the issue of the kilt wearing is her problem, not yours. If she asked, you might have been different, but she made a promise that she is powerless to keep, and she must learn that by whatever means necessary. (Kilted means really)