You do not need to ask anyone whether or not you can wear a kilt. (You would not ask permission to wear any other garment, surely?) But you do need to know what kind of wear is expected (formal, semi-formal, casual). As is said many time so on XMarks, the kilt can be dressed up or dressed down.
This.

People are under the mistaken perception that since it's 'the bride's day' that she can dictate what every guest will be wearing. As a GUEST, your responsibility is to pay respect and honour to your hostess by dressing appropriately for the occasion. The most you should discern about the wedding is the overall tone (casual, semi-formal, formal, etc) and modify your attire accordingly.

I'm confused as to why people would even bother to ask the bride's permission to wear the kilt. I just finished reading the thread by the gentleman whose wife is 'down on the whole kilt thing' and some of the responses were strongly worded. The gist of the thread being that the wife's attitude needs gentle readjusting. How is it that the bride gets more power than the wife?

I believe that it's none of the bride's business what you wear, but as a guest that you don't step over the line of decency. Do you honestly believe that a kilt is indecent enough that you have to ask someone's permission to wear it?

I tend to get carried away about stuff like this so I apologise if I have offended anyone.