Was any of that filmed and then entirely skipped over? Although, at the end, it looks as though all of the place settings are still there and untouched.
Why would there be dancing and cake first before the receptions?
Apart from being an afternoon event with dinner afterwards, I think it served the symbolic purpose of sex before marriage - you know sweet before the meal. That's why Altman focused on the dinner dialog with the reverend and the security man - two people "dedicated" to their task contrasted to the enjoyers.
"You couldn't be much further from the truth" - several