You need to explain to them that this is you and your FI's wedding and that this is something you want. Like finzup said if they aren't contributing financially they have no say in our they think you are "wasting" money. If you want to do it, it's not a waste of money. My Fi's mom cannot understand why I would want to ruin my wedding dress, and I just said because I do it'll give me beautiful pictures, wonderful memories, and I'm only wearing the dress once so I might as well have fun in it! Just explain how important it is and show them examples of what you want to do.