It seems as if there are basically two types of political arguments: the pragmatic, utilitarian argument, and the moral argument. The pragmatic argument says that if we do this or that, we will have the most happiness, satisfaction, or well-being for the most people. The moral argument says that even if this or that were to produce a generally happy society, it is intrinsically immoral and we should not do it.
A model of the moral argument is Ursula K. LeGuin's The Ones Who Walk Away From Omelas. In this story, LeGuin describes pragmatically successful society: while not saints or angels, its members are very happy and satisfied by most reasonable measures. This happiness, however, is gained at the cost holding a single child in perpetual misery, terror, pain, and degradation. LeGuin implies that this price is too high to pay for any happiness. While it might be true that all else being equal, more happiness overall is better than less, all else is not always equal, and sometimes the general good has to be sacrificed to a more fundamental morality.
Some arguments against slavery, torture, imperialism, and even capitalism take the same form. We do not have to analyze the overall well-being of a society that, for example, condones human chattel slavery; slavery is intrinsically wrong, and if we must sacrifice the general good to abolish slavery, so be it.
I am skeptical of the moral argument type on general grounds. Good moral intuitions without conscious utilitarian calculations are simply those intuitions whose utility has been historically established. It's not that we know slavery is "intrinsically" wrong, regardless of its utilitarian implications, it's that we know historically that slavery compromises the general good. In just the same way that we don't need to consciously do a lot of algebra and calculus to catch a ball, we don't have to do a lot of conscious utilitarian calculus to remember that slavery is a pragmatically bad idea.
But most moral arguments are more easily refuted. We need merely ask, is our conscience so gravely shocked by some action that we can justify ignoring or reversing the actions effects on the general good? In her story, LeGuin has to imagine what is nearly the most morally offensive action to give her argument strength; any lesser sacrifice, and most might well say that the general good in Omelas was worth the sacrifice. For example, in "If All Men Were Brothers, Would You Let One Marry Your Sister?" (analysis with quotations), Theodore Sturgeon's posits the utilitarian advantage of what is perhaps the second strongest moral prohibition, incest, and makes a reasonably persuasive case (with perhaps too much exposition at the end) that in this case the utilitarian advantages outweigh our moral intuitions.
I'm writing a lot about Libertarianism here, so I want to connect Libertarianism to the moral argument type. And Libertarians do use the moral argument: taxes, are an inherently unjust, coercive expropriation of property; taxes are inherently immoral, therefore we should abandon taxation regardless of its potential utilitarian benefits. I don't really buy this moral argument. Property is not an end; it is at best a means. I need food, water, shelter, etc. so that I am not distracted from my "pursuit of happiness" by hunger, thirst, cold, etc. So long as taxation does not interfere with the function of property to maintain my bodily integrity, it is hard to see it as at all immoral. And, at least in industrial societies, taxation by itself does not directly deprive anyone of the means of life.
But even if the the moral argument were not generally invalid, either typically or in the case of taxation, simply arguing that some action is "immoral" is a non-starter unless that action truly shocks my conscience. We are not a morally perfect society; by definition we tolerate some moral ambiguity and compromise. It is perhaps true that an ideal society would have no taxation, but we have to get to that ideal society. Unless that element so truly shocks the conscience that its continued presence is intolerable, we can't just abruptly reverse an important element of our social, cultural, and political constructions merely because it is possibly not part of an ideal society.
And, let's face it, except to a fanatic or an actual (or would-be) oligarch, taxation per se is hardly morally intolerable. I've been paying taxes for thirty years, and I do not feel the slightest bit of outrage or oppression at the mere fact of taxation, especially because I have a voice (albeit small) in taxation. Even if taxation were in some sense morally wrong, we do not live in a perfect society, and taxation is a moral compromise I can very easily deal with. Furthermore, I am no more sympathetic to Libertarians' moral outrage at taxation than I am at the conservative religious moral outrage at homosexuality. Even if taxation were immoral, it is not sufficiently immoral for its immorality by itself to justify its abolition.