I have had brides ask for a sugar free wedding cake. This is somewhat of an  oxymoron. Even if I use a sugar substitute in the cake mix, the butter cream is a combination of ingredients that include butter and powdered sugar. I always have an option of fresh fruit or sugar free jell-o for those guests who may not be able to tolerate  sugar. If it is the bride or groom, I decorate the cake in such a way that the slice they cut into has minimal frosting on it.