I think that's quite rude! It not only mocks the whole affair but it also shows you're not honest with your feelings or with the person you like (or you'd like to go to prom with) and shows you don't respect their feelings
Honestly, Manko is right. In your case, sinse they had that reaction, it would be prudent of you to apologise for creating that scenario and creating that misunderstanding (even though you planned that reaction if they reected your proposal). Show them your point of view, they may get mad at you...