Is It Tacky to Just Ask for Cash as a Wedding Gift?

One of our Bull family members needs some advice -- Frank and his fiancee are getting married soon. His fiancee wants to do the gift registry thing, but Frank wants to just ask for money instead of gifts.

They have been living together for awhile, so they have all the household items that one would normally put on a gift registry.

Frank would rather get money to help with the cost of the wedding or honeymoon. His fiancee thinks it's tacky.

Do you think it's appropriate to just ask for money instead of wedding gifts?