Betty's got the traditional view pretty well summed up. Once we've accepted a social engagement, we're committed to it, barring any emergency. "I changed my mind" isn't considered an emergency.
I agree that she does sum up the traditional view quite well, however, d@ting and general social engagements are different because they have different purposes. One could easily argue that were the OP were to keep her previously sincere, but no longer applicable commitment to the original date she agreed to, she would also be disingenuous because she has no intention of following through on the mutually-agreed upon goal of a "date".
The only way I can see to resolve this with utmost politeness is to inform the person that you are interested in/willing to stand by your commitment of social interaction at the previously specified time and place, but that you have changed the parameters to the pursuance of friendship, not romance. Frankly, I find this to be rather wasteful of time and effort for both parties unless there is a genuine desire for platonic friendship on both sides.
Theory vs. practicality. I don't think the OP did anything wrong. I think the OP's date was very mildly rude to express his annoyance in the way he did so due to the nature of the rel@tionship (or lack thereof) in question. However, I do not blame him for feeling that way, only expressing it to the OP.
For purposes of etiquette, social events aren't segregated into "might lead to something romantic" and "other." People are presumed to be interested in friendship, intellectual stimulation, companionship or an escort to a particular event, etc. and to be pleasantly surprised if anything further develops. (Again, I fully realize that some people's dating
conventions may not adhere to the above but this isn't a dating
-advice forum, it's an etiquette forum.)
Maybe the guy in the OP situation recognized the lack of sparks too but figured that even if the poster wouldn't make a life partner, she still might be fun to have dinner with, to see a particular movie with or attend the bowling tourney with. He's looking forward to Event X and planning his time accordingly and suddenly finds out the appointment is unilaterally canceled. Etiquette frowns on the person doing the canceling unless she is rehearsing "Good evening, Mrs. Obama, thank you so much for including me tonight," -- or if she or one of her loved ones is in the ER or the morgue.
Once made, a social appointment may not be canceled at whim. Imagine if you and your spouse have dinner with another couple, make specific plans to do so again and then when alone say to one another "I didn't really like them all that much and I can't see us all becoming firm friends. What say we cancel out on Friday night?" Is this situation to be exempt from normal etiquette rules, too, because you don't anticipate continuing the relationship
beyond the date you've already agreed upon? What about kids who accept party invitations and then confide to their parents that "Cindy is no fun, I don't want to be her friend any more and we aren't going to be in third grade together, anyway." Should mom say "OK, then, I'll just call her dad and cancel for you." Where does it end? There is a requirement in etiquette that, once we have agreed upon a social appointment, we are required to go and make the best of it, however dreaded the occasion may be.