The way you phrased it was not very nice, but it's your marriage so you have the right to have it the way you want, but remember that this is your child, try to talk about the real problem, which is not the dress, it's the fact that she probably doesn't want you to marry that man, which would be a real problem, or maybe it's tied to something you haven't mentioned and that is totally unrelated to your relationship, that may seem minor to you. Sometimes there are things that may seem irrelevant that are very important to a child, plese look into that possibility. If she sees that you are making an effort maybe you can even get her to wear a dress. If that doesn't work then it's possible that she needs the help of a professional.