If you don't mind advice from an older woman -

Your girlfriend's mother is probably not actually bothered about you wearing a kilt, or anything else - I think the whole thing is about territory.

I suggest that if/when you are to be in her house, then dress smartly in something you think that she will approve of, but when you are leaving ask the permission - preferably of the future father in law if he is around, to change into your kilt - or even just 'something more comfortable', before you go - so you don't have to stop and make the change elsewhere.

Try not to be confrontational if the answer is 'no' - but make it clear that once you are out of the house you will be changing your clothes. Don't think of doing it outside the front door (well OK, think about it, just keep in mind that she won't forgive you - ever - for 'showing her up in front of the neighbours' )

Many MiLs appear to regard their offspring's home as simply an extension of their own, you might have to work hard to maintain that future as yet mythical space as your territory.

You might have to forgo a kilted wedding - the wedding day is, traditionally organised by the bride's family - but you will need to make clear - as gently and as subtly as possible that that is the last day when you and your wife will be truly involved with her parents.

If they offer to book and pay for the honeymoon in a place of their choosing, it might be as well to accept. Do, though, point out that all you require is a bed, preferably a large one, and that the possibilities of sightseeing are going to be slight. Enquire if room service is available after midnight. Make it clear that wandering around interesting places, holding hands and gazing at the sunset is not going to be high on the agenda.

Give her your best grin and tell her that at least you won't be wearing your kilt most of the time.