Is it bigoted? Yes
Dawkins assumes to know Romney's beliefs and how he will act in accordance with those beliefs based on Romney's affiliation with a group.
Any argument stemming from Dawkins could offer to rationalize this statement would be invalid. However, invalid does not necessarily mean untrue. Rather, it just means that his argument does not logically follow. Thus, his statement is bigoted.
It is important to note that words carry baggage. Bigot is certainly a word which carries much weight. Is this statement narrow minded? Does this statement depict an "utter intolerance?" Moreover, when we think of bigotry, what comes to mind?
Look at the remarks that were made about Dawkins on this thread and others. Is this bigotry? How many in the United States would oppose a president who believed No God exists? Is this bigotry?
For me, this thread serves more to highlight more than just Dawkins' intolerance. I think a more interesting concept questioning what role core beliefs play in our decision making? what core beliefs do we want in a president? and why?
If we agree that core beliefs do play a role in decision making, then we would me remiss to not take those core beliefs into account when electing anyone to an office.