Asexuality is not some exceptionally rare, impossible thing.  Even using the common 1% estimate (which many people think is low), that’s 3.2 million Americans.  If you met someone who said “Hey, I’m from Chicago”, you wouldn’t tell them that you didn’t believe them, because people from Chicago are only about 1% of the population.  “You’re really from New York or California.  Statistically, that’s far more likely.“  That would be ridiculous.