We seem to be discussing two things:
1) Is a Birkin, because it is a tote and by nature somewhat casual, appropriate for a wedding?
Anything can be appropriate anywhere. If it's a beach wedding barefoot in the sand, a straw tote may be ok. If it's a rock climbing wedding on a rock face (I went to one) you may be carrying a backpack. If it's a Burning Man wedding, many guests may be wearing only body paint (been to one of these also). I'm assuming horseback Renaissance faire weddings require saddle bags.
However your run of the mill suit-and-dress wedding generally calls for a smallish bag. If the B works with what you're wearing, fantastic.
But 2) there seems to be an overtone also of "Is a Birkin "showing off"?", akin to wearing something very, very sexy at en event where the bride is supposed to shine.
Only two things can answer this - what's the milieu/is this, in this gathering, going to be an egregious display of wealth? And do you want to wear your Birkin to a wedding just to show off that you have one? If the answer to either question is yes, personally I would leave the B at home.
Since no answer will apply universally I feel it's all a bit
at this point