I, OTOH, believe it’s security. A woman wants to feel secure in her life and in her relationship with her husband.
Any sign that he is backsliding in his love and commitment to her is cause for her to fear, and that triggers those romantic fantasies that endanger the entire relationship with him.
Of course, his backsliding can be result of his own fantasies, which do not need to be romantic. They can simply be a fantasy about a life free of all those family responsibilities that pressure him all the time.
It's more complicated than that. A woman wants to be secure, but if she feels her man is with her only because he can't do better, her feelings turn into contempt. To preserve the relationship, the wife needs to feel that she's got a good guy, and she better work at keeping him.