I think that your mother in law is too hasty. If you had "a problem" when wearing your kilt, then the first thing somebody should have done was to have a quiet word in your ear. This person should have one of those who were flashed ( if that is in fact the case), or your mother in law herself. You could then have made whatever alterations to your posture that were necessary. To simply say ( and this was to your wife, not to you personally !) that you were banned from wearing the kilt, is just draconian.

I am not convinced that you have caused a problem. I think there is something objectionable in your mother in law.