Isaac can't help his own reaction and he's hoping at least its not a fast reaction. All he can tell is Scott's hand was now decidedly closer to places it shouldn't go and that wasn't causing problems by itself. He manages a nervous smile at Scott and Allison, perfecting an expression of 'absolutely everything is fine' while at the same time trying to put both Scott's and his own problem out of his head. Yes, this movie was good. Everything was a-okay.