I'm so glad to have come across all these posts about this problem.
I had a massage in a shopping centre only a few days ago. Here in Australia, as is probably common in a few other countries, there are these high turnover Chinese massage shops, which have several people working and being massaged in a small area, these shops are quite exposed to onlookers and passers by.
After receiving a neck and shoulder massage the first MT talked me into getting a full back massage as I was so tensed up. I accepted and he referred me to his co-worker who was a big Chinese bloke.
I go into the massage booth; a flimsy curtain is drawn, which still allows the public to watch as it does not cover the whole entrance. The big bloke asks me to slip of my shoes, but leave my jeans and shirt on.
He had towels on the table and covered me with a cotton sheet to massage me through.
While he was massaging my right buttock all the sudden I felt a warm sensation and in not time had a full erection coming on. I thought to my self, what the hell is going on here! No matter what I thought of I could not make my hard-on disappear. It was like he triggered a g-spot and had released a surge of warm blood directly to my penis. Before I knew it I had cum in my pants with my jeans on!
You could imagine my embarrassment as I had a huge wet patch on the front of my pants. I lay there thinking this is just not happening, it must be a bad dream. When the massage finally finished I bent over and put my shoes on, quickly grabbed a bag of shopping I had and held it in front of my pants, paid and left. Leaving behind a tell tale wet spot on the towel on the massage table.
I could not work out why I had cum in my pants, I'm a married man and not gay, the guy was a big unattractive person and I was a very public place.
I had to hold the bag in front of my jeans, dash in to Kmart and buy some new undies and jeans and get changed in the shopping centre toilets.
What a dreadful experiance.
I'm so glad I found all these posts; it has cleared my mind as I was very confused. I agree with the trigger spot theory in my case as this was a very quick arousal and ejaculation.