While I find this topic extremely interesting, I don't believe, psychologically speaking, that it can be properly termed humiliation if the sub is turned on by whatever is being done to them. In that case, I prefer to think of it as nothing more than another kink. While the sub may display signs of humiliation while a certain act is performed on them, such as blushing, as long as they also simultaneously experience arousal during that act, I don't think it's humiliation.
That being said, if another sub were to look in on this scenario as it was being played out, and found the act to be degrading, the "peeping tom" could call it humiliation, but not the sub on whom the act is being performed.
If, on the other hand, the sub on whom the act is being performed found it to be humiliating, to me, not only is that humiliating, it's also abuse. In that case, the sub should speak up and call a halt to the scene. That's what SSC and RACK are all about. This also points up the fact that individuals should talk before-hand about limits and be absolutely clear about what is permitted and what is off limits.
I realize this is a semantic argument, but the distinction, I believe, is an important one.