I remember watching a Billy Herrington wrestling video a while back where, once he beat the smaller guy in the wrestling match, he made the guy suck his dick. And as the loser started blowing Billy, his own dick went from totally flaccid to full mast in a matter of seconds. It stunned the hell out of me in the best possible way.
Yes, I love it in a porn when the bottom is actually hard while being fucked or sucking dick. It just makes is so much more believable and erotic! Those videos seem to be harder to come by, though.