Oh come on, everyone knows that The Moor is the best Opeth song to play at your wedding.
You should play the intro when your bride comes to the altar with her father.
"STOMACH SLASHED OPEN HE WIELDS HIS BLADE" etc. Better something wildly inappropriate than an über-sad/über twisted soft Opeth track that'll just be SOMEWHAT inappropriate.
Opeth is a band that talks about death, hate, murder, rape mixed to love. If you find it representative, that's your choice. Musically, I'd go for Harvest too. Too lazy to find any other song.
Getting married next May. I'm going to try to slip Benighted in during the reception while people are drinking wine. Lyrically, all of the other ballads are inappropriately sad.
BTW, we are Ipoding our wedding instead of paying a DJ (one guy quoted $2400). We are buying a PA that has an Ipod dock and mic plug-ins. It even has karaoke for when the party is winding down.