Yes, in point of fact, they are. In a *host* of ways.
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They unduly and irresponsibly elevate infantile innuendo to the same level as systematic empirical observation and data-based theory. 
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They deify raw sensory experience (Pre-/Concrete Operations) at the expense of abstraction (Formal Operations), and denigrate the latter as mere herd obedience.
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They contribute to the spread of a spiderweb of cracks throughout the precious and precarious crust that separates us from the recently (and imperfectly!)-encapsulated magma of rampant magical thinking and superstition. Try to imagine how they would use our modern tools to wreak even MORE havoc than they have always done. 
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ALWAYS.
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These Flat-Earthers are viruses in the very *nuclei* of the Noosphere. They threaten the intellectual and philosophical (and --if history is any guide-- the PHYSICAL) integrity of the  ecosystem into which my son will all-too soon go forth.
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This is UN-FRAKKING-ACCEPTABLE!
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They bloody harm us ALL!