Molon Lame

Thermopylae comes up a lot for two reasons: 1) Xerxes asked the Spartans to give up their weapons, and 2) Zack Snyder's 300 gives members of the "patriot" movement hard-ons that could burst a carbon-fiber condom.
Guess what, the attributed utterance of "molon labe" may not have even happened at all!  According to Stephen Hodkinson, BA, PhD, FSA, Professor of Ancient History and Director of the Centre for Spartan and Peloponnesian Studies at the University of Nottingham: the phrase didn't appear in extant writings from close to the time of the Battle of Thermopylae.  We only know the phrase from the writings of Plutarch, which come from about 580 years after the battle:
Finally, some comments on the “molon labe” phrase ascribed to Leonidas at the battle of Thermopylae in 480 BC. It does not appear in Herodotus’ account, written in the later 5^thcentury, which records the witty sayings of another member of the 300, Dienekes. Neither does it appear in accounts of the battle deriving from Ephorus, who wrote in the 4^thcentury and who himself drew upon a contemporary poem about the battle by Simonides. To my knowledge, the only appearance of the phrase in all the ancient evidence about Thermopylae is in a work by Plutarch, writing in the early 2^ndcentury AD: theApophthegmata Lakonika(Sayings of Spartans), which is part of Plutarch’sMoralia. It is no. 11 out of fifteen sayings ascribed to Leonidas.

Professor Hopkinson later clarified in an email:  

As far as we can currently tell, many of [the Spartan sayings] were probably invented in the late 4^th or early 3^rdcenturies BC, at a time when Sparta had ceased to be a major international power and became instead an attractive source of moral examples for the new and rising Hellenistic schools of philosophy. However, the late 4^th or early 3^rd centuries BC is still 150-200 years after Thermopylae, a long time after the event.

In sum, the historical authenticity of the phrase “molon labe” is uncertain.
Of course, you can have all the facts on your side, but no one will care if it isn't an interesting story. The ignorant will chose an interesting story to the facts any day.
See also:

• Did Sparta Exercise Arms Control In Its Society?
• “Molon Labe” Was Uttered By An Arms-Controlling Tyrant
• 6 Things Gun Lovers and Haters Can Agree On--especially item number 2