I've seen a lot of people get dress wedgies when they're wearing thongs or going commando, so that was my first thought. I can also see thongs being an issue if the dresses are knee-length - having had a couple Marilyn moments myself in my younger years, I almost always wear bike shorts under my skirts/dresses now! But it really seems a step too far to dictate what kind of underwear these girls should be wearing at the wedding - they're old enough to have an idea of how to dress appropriately.