Thinking back on this subject, I came up with 2 other times that were humiliating, but not so much as the first example above. The first one was cumming on my wife's leg just above her ankle. I held my dick against her leg, and began humping. Magically, in about 15 seconds, I unloaded while my wife called me a pervert for cumming on her leg. It fit me well, because legs are my favorite part of a woman right after pussy.
The second time was while wearing my chastity cage. I found I could get excited by shaking the cage, even though I was prevented from getting an erection. I hadn't cum in over 2 months, so after a couple of minutes of shaking the cage I had a cum without touching my dick while still completely restrained and limp. The cage was left on and wasn't removed for another couple of months. I never tried to cum by shaking my cage again, because the point of wearing it was to stop me from excessive masturbation.