Antony Flew used to be fond of this example:
A: No true Scotsman puts sugar in his porridge.
B. But what about Mr. Campbell here. He's as Scottish as can be, surely.
A: He's not a true Scotsman.
B: Why not?
A. He puts sugar in his porridge.
Sometimes I could swear that I hear this line of argument:
A: No true scientist accepts intelligent design.
You fill in the rest of the dialogue.
Tell me this isn't so.