They'll find out the marriage isn't legal in the most absurd way possible, no doubt.
Like, Bart & Lisa will start a lemonade stand, but get bored with it and somehow trick Homer into taking over when he's supposed to be at work. While making the lemonade, Homer creates a *new* flavor of lemonade that inadvertantly ends up triggering an allergic reaction in one of the secondary characters. Wiggum arrests Homer for attempted manslaughter and takes him to jail, where he meets the priest who married him and Marge. End first act.
Act II, Homer takes the priest home to stay with the family, they bond. Priest and Rev. Lovejoy get into a Kill Bill parody that takes up a good chunk of act II. It's revealed the priest if a fraud and the marriage is invalid. End act II.
Act III, Homer and Marge fight, priest mysteriously vanishes with no explanation or worry from the other characters. Homer gets kicked out. End Act III.
Actv IV, Homer comes back, they make up, and they cram the wedding into the last two minutes of the show.