From the My Pocket Book Only Has Eyes For You department:

Canada.com posts an article about infidelity. Not the marital kind, but the financial kind...Which is also sort of a marital kind. Financial infidelity is when one partner hides or lies about their spending to the other. One theory why people do this is so they can assert one's independence within a partnership. Business coach Fred Horowitz says, "hiding spending from a partner suppresses self expression and hurts the relationship." I think there is a simpler explanation. How about because guys don't want their wives knowing that they spent half a paycheck on Spank69TV to channel their natural thoughts of more serious forms of infidelity in a healthy way? Or what about buying gifts for your spouse? Shouldn't that be kept on the down low for a while? At least until the gift is given? Even after the exchange takes place you may not want to share what you spent on it. I was always taught that to ask how much a gift cost was to be extremely rude. (Canada.com)