My feeling is that you can be a "fundamentalist anything" -- when there's no room for argument, discussion, understanding, difference, tolerance, or acceptance.

Well, listen to William Lane Craig as an example of the religious fundamentalist and listen to Sam Harris as the opposite and tell me who is the more ambigous.

These are small-group survival dynamics, and they work well as a default operating procedure if one wants to help ensure the safety of a group against uncertain or imminent external dangers. In that case, intolerance of ambiguity is utilitarian. We wouldn't have made it far as a species without the ability to quickly classify, judge, and reject what is unfamiliar as potentially unsafe. It's risk management. Organisms that can't practice sound risk management might walk right into the mouth of the predator.

Some people have a really rough time operate beyond these characteristics, preferring what has been proven to be familiar, comfortable, and reliable - avoiding and rejecting things they see as potentially risky. That's okay. We can cater to their needs just as readily. But if we want them to move beyond, the trick is to teach them how to safely step out of their comfort zones and embrace the unfamiliar. Not easy!