From a strictly rational standpoint, you can't reject something out of hand just because it's repugnant. But you can reject it if it makes no sense or is of no practical use.
Fundamentalist theology doesn't make sense, because here we have a perfect god that fucks up his creation, then blames the creation for his own fuckup. There's also the fact that there's nothing in the world or human mind that directs anyone towards a specific god, or that god's goals. It also forces you to believe everything in the Bible literally, rejecting all scientific evidence to the contrary.
The question "what if he's actually right" can be applied to literally every belief system ever made. If I was to say the entire universe was made yesterday with everything created in place -- memories, light on its way to places -- to create the illusion of age, you can't really reject that out of hand, either. But what's the point in believing that if nothing useful comes of it? An open mind is one thing, but to consider the idea of Santorum being right, without considering every version of reality anyone could possibly pull out of their ass, would be giving him way too much credit.
I don't think you understand how this woks, how can you know the final solution is a bad thing unless you think about it first?
Just assuming things are wrong without actually thinking about them is totally a fundie thing to do.
Put that way, yes. But we shouldn't always have to show our work. That would be like making someone show how they got from 5+5 to 10 in a class on partial differential equations.