I don't think it is unreasonable at all and surely I'd react in the very same way.
Did you at least receive an apology (even if it would not be enough)?
The worst part about it was doing it behind your back.
Asking for your permission to date her would have been the very best thing to do, but even something, "hey, I really like this girl so I'm going out with her, I hope it won't be a problem because I'm doing it anyway" would have at least been honest and not disrespecting