I've been caught jacking off twice when I was a teenager; once by my mom, and once by my dad.... it was humiliating and I couldn't finish. Never in the middle of blowing my load though, THANKFUL-FUCKING-LY, that would be the worst.
As for women who get pissy when their boyfriends masturbate, why do they seem to have a problem with men getting off? So many of them consider men jacking off as "cheating", and, as mentioned earlier in the thread, they think it looks stupid and ridiculous. Some even think of male sex toys as infinitely dirty and grotesque. As a gay guy, I don't get that. I'm single right now but I would LOVE to watch my partner jack off, and if I walked in on him doing it, I would think that was really hot and I'd quietly observe.