Haha, love the disrespectful comments!!
Why on earth should anyone respect the institution of marriage through the medium of clothes?
Ridiculous, wear what you want, what makes you feel good, I presume they invited you because they like you rather than because you'll look all swanky on the photos! I went to a black-tie ball last week wearing leather leggings and a silk shirt whilst all other women were in frocks, I can't tell you how many compliments I got (my grey quiff was also the source of much admiration), fuck the rule-book, throw that shite out of the window, someone wearing what they want and fucking owning that is way more chic than following some outdated code.
Wear the jeans with pride, stride in, people always look their best when they dress in a style that suits, rather than what's 'expected'.
The last two weddings I've been to I've worn a YSL smoking suit, I ain't wearing no frock!