I DONT think the looks/Tznius connection is the main cause/root of deterioration of most marriages. I think over time, people just get too comfortable with their spouses and dont treat them with the same care/dignity they did when they met. I think lots of people are unhappy with their lives in many areas, and their spouses are the most convenient scapegoats (no matter what the spouse looks like).
I think the blaming issue is a major issue, meaning one spouse blaming the other for whats wrong in their life, either outwardly or subconsciously. Imagining that if they would be married to someone else, their lives woud be happier and more problem-free, caused by frustration with their lives, which marriage is only one aspect of (children, career, social standing are some others).