Well...it's a good thing your fantasy wasn't about you with another woman...rite? I mean, if it could be any worse...that would be it?
Reassure your wife that some fantasies are not meant to become a reality...hense the name, fantasy. And what better person for you to share yours with than the person who means he most to you?
Ask her...would she rather have you confide in someone else?...I think not. Give her some time to absorb the 'surprise' of what you told her. Patience. She'll come around. And perhaps encourage her to share some of her fantasies with you, as well? After all....we ALL have them.
Doomed? ...I highly doubt it.
Hope that helps some.
Shangrala