I think there is an amnesia that kicks in when you have children. I'm not talking about your daughter having a child, I mean you. Do you not remember what is like to be 16 and your body constantly bombarding your brain with messages that say "must...procreate...must...procreate..." If the boy was 16, would you be any less upset? Would it be rape then? What difference does it make what age he was?
I think you have lost your daughter forever. If you was 16 and you was truly in love with a girl and your parents forbid you to see her, would you just say, "Fine, you are the wise and powerful parents and I shall obey your wishes?" When you are 16, you are in my opinion an adult. You can make rational decisions and live with those consequences.
Just for kicks, take a look at
www.ageofconsent.com and check the laws not only in the different states in the US, but in different countries. If it was universally accepted that 18 was the legal age of an adult, then there wouldn't be such a variation in the age in which a person can have legal consentual sex.