Both of you are right; one shouldn't assume that all of the stories are true because many of them are.
It works both ways, though. There is still that chance that the rest of the bag is filled with green marbles. One’s faith is exactly what may determine what is actually in the bag in the first place, eh?
However, lets say you do draw a red marble after three consecutive green marbles. One had assumed the whole bag consisted of green marbles, but now realizes it is not. However, that isn't necessarily a bad thing. That is, of course, assuming a green marble represents a true story and a red marble an untrue story.
Personally, I believe many of the stories to be untrue in the Bible. By that, I mean that the events did not actually occur. However, I believe these "stories" to be an example for us to follow. Take Adam and Eve, for one. I believe that is a story for us to understand how the earth was created. I don't believe Adam and Eve were both actual living beings. However, that is a lax interpretation of the Bible. I'm not saying what I believe is right, nor am I saying it's wrong.
What I am saying is; so what if you draw a red marble? Even if it's not green, it will still teach you something, which is all the marbles are really supposed to do.