I booked this through a travel agency, and they are equally devastated. Their Holland America rep is also floored by this and is working to try and get this changed. Our travel agent has been on several HAL cruises with her husband when they were underage and there was never any problem because they were married.
I understand I might have to make some changes, but it doesn't change how angry I am and if there is a small chance something can be done. I have saved for over 2 years to go on this cruise and take that much time off of work. I don't think that should matter though; its the principle that a young married couple can't go on a cruise because one of them hasn't turned 21 yet.