So what does this actually mean? You see a girl, she looks intelligent and rational, you learn that she's a conservative Christian and you go "Oh, she isn't intelligent at all, my mistake"..?

Not a very charitable interpretation. How about this instead: If someone is a conservative Christian then that fact makes it less likely that person is rational.

Similarly: If someone is deaf then it is less likely that they are a great pianist.

I can affirm that statement and still believe that Beethoven existed, without implying any insult to Beethoven.