For what it's worth....
In the past when I was uke for my instructors, I left the fate of my well being in their hands. I would lend them the use of my body to demonstrate any aikido techniques and put my trust in them to treat my body with care and with a bit of tenderness. Alas, that was not to be.....
At this point, I would leave the dojo. Seriously. If I tap and the reaction of the head instructor is not to honor that, that's it I'm gone. I'd also consider legal action.
But it bugs me, as an afterthought, that without an explanation, he would think of me as selfish and arrogant. What do you think? Do I owe him an explanation?
A number of issues here.
Personally (and I think I'm a minority here), I think this is an inherent problem with the common aikido training method. With no dynamic method there are unclear guidelines as to what resistance is acceptable. I don't see how that can be resolved without a clearer framework in which to train.
As for the explaination, it's probably too late now. Could it be that they were just having a rough day and their frustration had nothing to do with aikido, the technique or you? If this is standard behavior for them, have other in the dojo had similar problems with this individual? Have you ever had this experience with anyone else? Has anyone else had this type of experience with you? How "big" a deal is this? Will you be able to train together in the future?