Maybe Ann cheated on his husband simply because....

Bill was younger, more fit and more attractive than his husband? Maybe James was a good husband but he was a fat unattractive middle aged man and you know it's not only men who like fit attractive women, also women like fit attractive men. She simply was tired of sleeping with a fat and out of shape man. Anyway you know in the movies when a man cheats on his wife is a pig but when a woman cheat on his husband she is has her good reasons or she is misunderstood...talking about double standards.