I know they don't allow you in the park with attire that looks too princess-ey, wedding or not. But also, my wedding dress isn't a poofy gown, but it isn't great for roaming around a theme park either. I was told recently by our wedding co-ordinator that the best plan would be to change into park attire at the venue once the cake/champagne celebration is done, and store our wedding clothes in the lockers they have at the entrance of magic kingdom. You rent a locker for 4 or 5 bucks, I think. They don't have a service to ship our clothes from the venue back to our hotel. Grrr.
We were there in Oct 2015 (we got engaged there), and we did see a bride in Magic Kingdom, wearing a very simple white, knee-length dress. (With her stockings rolled down as she was clearly done with the formal part of her day!) This was just in the part of the evening when everyone was gathering for the parade and fireworks.
We picked Magic Kingdom to do on the afternoon of the wedding because that IS what most people think of as Disney World, and it has so much classic Disney. But also that a lot of our family are not into WDW at ALL. So one afternoon is minimal for them to have to put up with, but also to maybe show them what the Old Disney Magic was all about. That, and the Electric Parade and Wishes Fireworks to end our special day.
But the problem with that, is there aren't that many great sit-down restaurants for a big family dinner in Magic Kingdom. Be Our Guest is the ONLY one in MK that serves alcohol at all. But is is too themed and character dining is not great for the rest of our family. A couple others are more lunch-friendly.
I would suggest going to the WDW Dining page - you can click on the lists at the top to filter by specific park, and then filter for what restaurants accept reservations, and see what suits you.
If you have fussy family, there are TONS of restaurants in EPCOT to choose from. The Coral Reef, where I want to go, is under the aquarium, and you can literally watch fish, sharks, and sting rays swim past you while you eat. Plus amazing food. I'm NOT a foodie, and I took pictures of the food there. Also, lots of nice restaurants in Disney Springs, but that means being outside the parks. A lot of brides do make that choice. For us, Disney and Disney Magic was part of us getting engaged, and we want the park magic to be an intrinsic part of our day. Other brides make the parks their honeymoon after the big day.
If you haven't been there yet, check out
http://www.disneymooner.com Forums are here:
http://www.disneymooner.com/forums/index.php
Once you pass the five post requirement and request access to the full forums, there is a TON of useful information.
Feel free to PM me if you want any information about the escape wedding planning too!