@DestinysDad,
First DestinysDad, I have to say this; It is not about the dress. It is not about objectification of women or about image disorders.
From reading your post, it sounds like part of you understands this.
I can understand your feelings, and I think it is a good thing to communicate them (as long as you accept that the issue is your feelings). I am skeptical that one dress in a fashion show would cause issues in a relationship unless the issues are already there.
The bottom line is that what she wears is her choice. You have several choices. I think communication in a relationship is a very good thing; I absolutely think that you should express your feelings. It seems to me the best response is that after you have expressed yourself you accept her and let it drop. Of course you could leave her over this (it doesn't sound like you want to do this); but, the idea of damaging the relationship over this (i.e. living in a less than ideal marriage) sounds like the worst of all options. On the other hand, you could decide to put your feelings aside a give her the support she is asking for (and yes she has a similar range of options).
I don't think your feelings are atypical. In fact, in many marriages they are common.
The only suggestion is that you should recognize that your feelings are really the only issue here. Express them, accept that this is her decision, and then do what you can do to have a good marriage.