Hot story. Seems like something must have happened before between them as per your story your GF was very accepting of all the initial touching and let them both continue - her only reaction was to look at you, looking for approval, or at least not disapproval? And the older couple must be swingers or have played with others together before.
Have you talked with your GF about her playing with others again? Or at least role playing, like going into a bar separately and seeing how fast she can get picked up? Guys must be hitting on her all the time... Very sexy girl...
