Sorry Joab, but I'm with JKS here. From what I can gather, you were (at least in the instructors eyes) telegraphing your motions, and he chose one particular action to demonstrate the opening you were leaving, and the danger it represented. Now, if you took offence at his chosen target, that is one thing, but unless you were being singled out in front of the entire class, and then kicked in the posterior while they watched and laughed, I would say that your humiliation is your personal take on the action, rather than anything else.
That said, it has been remarked that all communication is in the perception, rather than the intention, so I would have advised to talk to the instructor in question and express your discomfort with his teaching style, then decide if you wanted to continue or not. But you say that this was "many years ago thousands of miles away from my current location", so I am unsure what you expect to get from dredging it up for yourself again now.
So you know, though, within a number of systems (including my own), there are kicks to the buttocks, as well as other strikes and pushes/pressures, so if it just comes down to a discomfort with a part of your (and other peoples) anatomy, then you may need to realise that a good part of martial arts is going beyond what you are currently capable of, and that includes dealing with personal beliefs and values, as well as fears and limitations. Oh, and hitting a student to demonstrate a lack in their technique is nothing new, nor something to be avoided in training. In fact, it can sometimes be the best teaching method for certain situations and personalities, and it is one that I use myself as required (as JKS said, not maliciously, but when it is the best choice). If that is not understood, that is fine, but realise that an instructor is coming from a place of experience that the students do not yet have.
Oh, and re-reading that last post of yours, I am assuming that every second paragraph is your responce? If so, then I would say that your take on JKS' teaching as using humiliation is off. You simply don't understand the teaching methodology here, which is fine, but you are reading into it something which is not there. So I wouldn't dismiss him as a teacher due to your misunderstanding, he specifically gave examples as to when he would use "physical encouragement", and there was no indication of humiliation involved.