Why does my dad insist on saying stupid sh--?

Say to him "Well I guess we'll just have to agree to disagree on that Dad." If he tries to probe you just say..."I dont want to fight with you Dad. I just can't agree with you on that." And do not fight with him. That way he knows that you disagree and you have asserted your right to speak your mind. But you dont have to get into a knock down drag out fight with him. Believe me, I had a stubborn conservative religious father, and I fought A LOT with him. It wasn't really worth it in the end. Just say what you mean and back off it.