Although most people wouldn’t think that financial infidelity is even in the same ball park as cheating with another person, it can have a similar impact on the relationship. Financial problems are still one of the top reasons for marital breakup, so getting on the same page and trusting each other around your spending is important. Once trust is broken, it needs to be built back up over time. Be open with one another and keep your finances and your relationship on track.
source: http://www.nefe.org/press-room/news/financial-infidelity-poses-challenge-for-couples.aspx