Me's letter regarding Peach and Bowser's wedding in Super Paper Mario is an easy one to answer..
There is a line that says something like Until your Games are Over which is concurrent with the phrase until Death do you part. Dimentio sent the party to the Underwhere, and everybody else in the game assumed them dead, so it was at that point that Peach and Bowser's Games were Over, thus nullifying the wedding.
Yes, I did use spoiler tags. There may be someone who has yet to play/finish Super Paper Mario.