At most of the weddings I partecipated, it was actually implicitly forbidden to wear too fancy a dress/tux, because that would have detracted attention from the bride/groom. (I do not actually endorse this line of reasoning either.)
I think that, unless you have a very, very conservative family, you wearing something other than a dress will most likely pass unobserved, rather than cause the wedding to be ruined forever.
(It must be noted that I am somewhat defiant of social conventions.)