I'm sure someone's touched upon this already, but I'm too lazy to real all the posts in here.
Marriage is a commitment. Two people committing publicly their love for each other and their dedication. You could even take it further if you're religious, and say you're declaring your love and devotion with God as your witness. Pretty powerful stuff, not to be summed up with a simple piece of paper.
Oh, and the tax break.. sweet tax breaks.
Trigger
Warder to Spettio
FEAR.
Mike Mackert stalks you on myspace.