Why do couples feel that once they get married they're never gonna have the hots for any other person?

Why don't people realize that we're all vulnerable to temptation at any given point of time, married or un-married and hence try to be cautious to prevent anything that has the potential to break our relationship such as, provocative dressing, or attending wild parties, etc etc...Don't you think most of us take relationships for granted and do what we want thus causing America to be the country with the highest divorce rate in the world.