I believe the parent is presuming a higher base level of rationality in GP's "gut" than that of the population-at-large. While I'm not a parent, I'm certain that my "gut" would tell me that vaccinating my child is the opposite of wrong, however unpleasant for the child it might seem at the time.

Of course, having watched friends become parents it's also quite clear that becoming a parent changes one's brain in some fundamental ways so it's possible you have a point ;)