[krapp]

>Because absurd opinion like "the earth goes round the sun" and "there's nothing wrong with being gay" were once "knowingly false and inflammatory" according to the overwhelming majority of people.

... therefore we need to keep protecting the speech of people who still believe the world is flat and that homosexuality is an affront to God just in case?

No, I'm sorry, that's an appeal to emotion disguised as rationality. We don't need to keep knowingly false, disproven and regressive ideas around any more than we need to keep arguing the merits of miasma theory, phrenology or the luminiferous aether. To say otherwise is to claim that truth cannot exist, merit cannot be measured and all ideas are equally valid... a premise already discredited by the value judgement made by your argument.