Please forgive me for answering a question with a question, but let me put it this way.
Strictly speaking, following your logic, it would be unreasonable to anything more than agnostic (either way) on anything that one could conjure up in one's mind. For instance, if one of my friends told me about a giant polka dot balloon, made out of steel and that eats lions and that it's floating in a galaxy a few million light years away, would I be basing an assumption that it doesn't exist on anything more than faith? Obviously, I would concede that I can't know it doesn't exist, but I would put a very low probability on it's existence.
I'll reiterate my original question, are you an agnostic regarding Santa Claus, the Flying Spaghetti Monster and fairies?
Reasonable-ness != Rationality.
It would be irrational to make a statement about the existence of the example balloon, given that you've no evidence for or against. If you were to fly to example galaxy and observe its absense, then you'd have evidence with which to make a rational statement.
However, that says nothing about what a 'reasonable' statement is. Whether something's reasonable or not is entirely subjective - I could say that it's reasonable for me to assert that you're 13 feet tall with pink hair, which is why subjective terminology is not particularly useful when discussion the rationality of assertions about the universe.
As for Santa, per my definition of Santa (fat man in red suit who visits every Christmas Tree on Earth on Dec 24th), I've conducted an experiment and gained evidence that Santa doesn't exist. You're free to conduct that experiment as well to draw your own conclusions about the existence of Santa, as defined above. As for the FSM and fairies, I've no idea whether they exist or not, and I've never attempted to define these entities well enough to bother testing whether or not they exist, so I guess I am agnostic towards these.