i actually think for some, marriage is the kiss of death for a relationship.
you hear stories of people who had been together 15-20 year, yet split up 6 months after marrying.
i think the problem is 'ownership'.
people tend to be on their best behaviour at the start, and keep up the 'making an effort' for fear that their partner may leave them - this awareness that it could easily happen keeps them attentive.
however, once they marry, they feel, perhaps subconsciously, that they have 'won' and now no longer need to make and effort to impress, because they have already got what they wanted and they now 'own' their partner.
they can now relax and let the 'real' them come out, the side of them that they had been keeping quiet.
they know that although the parnter could leave they are less likely to if they agreed to marry and have to go through a lot of pain and trouble with a divorce.
cynical i know, but thats sometime how the human mind thinks.