After reading the entire thread again, I think one point MM meant to make, but missed, is that a ball gown is not appropriate in the day time, and a full-length strapless dress in silk or satin is a ball gown. In a light unconstructed cotton print, it might be a beachy sun dress, but in candlelight satin with beads and lace and boning -- it is a ball gown. And what ball happens at eleven in the morning or four in the afternoon? Unless it is a formal evening wedding, I think a ball gown looks like a costume, not a dress, and that is what she disapproves of. I think it is the appropriateness of the style for the time of day and the nature of the event -- not "wedding" so much as "church/temple", "registry/JP's office", "grandmother's garden" followed by "brunch," "tea", "BBQ", "cocktail" or "ball" -- that she is reacting, not very eloquently, to.
Wear whatever turns you on and doesn't offend the rules of your venue as far as I am concerned, and maybe a full-on strapless, boned, beaded, crinolined, spangled and laced "bride costume" is exactly what you want to wear at brunch. Enjoy! I think MM would prefer you wore something more brunch-like and that is what she is trying to get across.
I wore an old-fashioned style of dress native to my ethnicity at my temple wedding in the Old Country and both the US receptions. It was definitely a "bride costume," and I couldn't have got out of doing it no matter how I tried. At the "make it legal" stop off (long story) I wore a casual dress I had with me that was appropriate for a JP's office and, later, dinner at a nice place. The Prof wore slacks and a sportcoat, ditto. To haul out and put on the Native Bridal Costume and his dinner jacket (there is a Native Groom Costume, but he wouldn't do it, and I agreed) would have felt silly to us, especially in the early afternoon in a JP's office, and I think it is that to which MM really speaks. FWIW