It is only when someone cannot defend his opinion, and is not interested in believing the truth, that he will attempt to stifle discussion with good manners. Those who take religion, politics and sex seriously do not adhere to the general prohibition on discussing these topics. And they do not take offence when they are shown to be wrong.
If you start to feel during a discussion, that you are not so much incorrect as insensitive, then you are probably dealing with a respectable bigot.