Dude, any person who does not believe in the literal translation of the bible (about 2/3 of the American population) is going to "marginalize" in some way or another the bible when talking about it, if you're looking at the bible from a literal point of view. You don't need fantasy stories to do it. More importantly, any person who does not believe in the bible being at all true, or being very distorted, is going to do so with or without fantasy stories.
It's just as easy for somebody to point to a picture of Starry Night and say, "Well, the diegesis in this piece is unlike anything you've seen in real life, so the diegesis in the Bible could be equally unreal." Should there be a stance against all still art, too? And, really, in all movies--as any person who has studied film will tell you--everything from the basic appearance of movement to the editing affects are facets which appear to be real despite the fact that they are illusions. So, again, it's easy to say, "Films look like moving pictures, but they're not--everything in them is fake. The bible probably just looks like a historical outlook on Christ's teachings, but it too is fake."
Nearly ANY art can be construed to compare to the bible as "unreal." Not only is that logic inherently flawed because it attacks one facet of creation where all facets fail, but it is also flawed because even without the arts these comparisons would STILL happen, so the art is, in this case, an unnecessary tool instead of the cause. In fact, like you said, it would be just as easy to compare the Bible to facets which stem FROM Christianity, such as Santa Claus or the Easter Bunny.
Any person who has to defend their belief by attempting to take away ways to question it, rather than addressing those ways, either does not understand his or her beliefs well enough or is believing the wrong thing. Obviously, with Christianity, they are not inherently believing the wrong thing. Darx, if your argument is, in fact, a stance any person takes, perhaps these people should spend more time trying to understand why they believe what they do, and less time trying to remove the beliefs of others from their surroundings. It seems to me, however, that you are misunderstanding the stance some people are taking.