I think the article does an excellent job exploring the various reasons, definitions and scenarios, but ultimately I think it’s a lack of total honesty and mutual trust and respect of both partners. When you’re in tune with your spouse or partner and can communicate openly with them, you are able to clearly identify and discuss whatever may be the real cause of dissatisfaction or dissociation.
Marriage does ask a lot of each individual partner, and it’s easy to get disillusioned or settle into a routine that becomes old quick. It is also a test of character, teamwork and maturity. Many people aren’t ready for that level of commitment when they go into their relationship(s) and that leads to unfaithfulness.