One of the most common mistakes is assuming that political opponents are ignorant. If only they had the right education; consumed the right media; or had the correct experience, they’d surely see things properly. The error is believing that we arrived at our values through reason. Values like hierarchy or equity are adopted by a complex process of disposition, emotion, and experience. Reason may be a component of this process, it may be a value unto itself, but it cannot support values.
Even simple moral claims like “it’s wrong to steal” cannot be supported by logic. Give it a try and you’ll come up with arguments like: “stealing is wrong because it harms the victim”. But you’ve not solved the problem, just pushed it back a step because now you have to defend the claim “it’s wrong to harm”. You cannot use observations about how the world is to calculate how it ought to be. Justifying moral claims with other moral claims dooms you to circular reasoning and infinite regression.
For those of you clever enough to argue deontology or utilitarianism, I’ll point out that these systems are ethical. Only concerned with how one should behave; helpless to prove something just or wicked. The moral principles of deontology and utilitarianism are assumed, not proven. Both systems will endorse ridiculous, intolerable, and outrageous actions in particular circumstances.
Objective morality probably doesn’t exist and has never been justified.
I don't necessarily disagree with many things you've said here, but you have ironically presented it as absolute fact, even going so far as to define what you consider to be the only clever comebacks to your own opinions. In other words: pot, meet kettle.