From an article posted on npr.org:
Marital infidelity is well-known, but financial infidelity might actually be more common.
Every couple might differ in how it defines financial infidelity. Typical cases often involve hiding compulsive shopping or gambling debts. In others, a spouse might siphon off cash from the family’s funds for a secret purpose. Either way, when the deception is exposed, it often evokes feelings of betrayal and loss of trust that can lead to the dissolution of the relationship.
Read the full article for more information on financial infidelity.