> The argument could be made that we should all be rational beings of pure logic, incapable of taking personal offense or being emotionally slighted or discouraged by such judgements. But we're not.

Which is exactly why it is fine to call an idea stupid. Everyone knows that people are fallible, so they know that your idea can be stupid without you being stupid. You taking that personally is stupid though, as it implies you think it is possible to be infallible.