Except, at one point in time, saying "Black people and white people should have the right to marry each other." could incite violence. People could, and did, get hurt for saying those things; so should we have shut those speeches, rallies, protests, and marches down because someone could get hurt?
The hard part about freedom is fighting for the other person's right to say things that you think are sick and twisted. No one fights because they think $DEITY is with the other side - everyone thinks they are "in the right". The Constitution has freedom of speech FIRST for a reason.
Of course, if anything goes beyond "speech" - violence, oppression, etc. then it is time to drop the hammer. However, if it remains speech, then it should be protected.
P.S. - It is legal to shout "Fire!" in a crowded theater. It is the desire to incite violence or mayhem that makes it illegal. (Ex: There could be a real fire.)