'All My Exes Live In Texas'? For subsequent marriages anyway.
Sreiously though, for an outdoor wedding, for slightly untraditional folks, a slower than average rendition of Pachelbel's Canon is pretty hard to beat.
And the Dies Irae is part of the Catholic Requiem Mass. Mozart and Verdi have two of the more famous ones... Or at least two of my favorites.
Canon (or Kanon) is a simple classical composition similar in structure to a 'round'. It's just a lovely, if trite, instrumental - Pachelbel's Canon is probably one of the most famous classical pieces.