I think the common vein you are mining is the utopian or the forcibly consistent nature of some of the libertarian philosophies. Humans and their workable cultures and institutions are organic, live in an historical and cultural context, and are not wholly rational & divorced from history and tradition, the way much l-tarian philosophy is.
I don't think I completely agree when it comes to classifying Libertarianism as utopian. Libertarian philosophy is no more utopian than any other political philosophy. It is true that Libertarians believe that some things that are being done today are wrong and that individuals and society as a whole would be more effectively and more morally served through the application of the Libertarian philosophy. However, that is not to say that Libertarians necessarily believe that if they snapped their fingers and the world were Libertarian that there would be no problems any more than Republicans believe that if every office were held by a Conservative Republican that there would be no problems. Members of every political philosophy believe that to the extent that their doctrine is implemented things will be better, more efficient or more moral. If that were not so, why fight for their side? Aside from rent seeking, I mean.
That said, you are spot on when it comes to tending to ignore historical and cultural context. For instance, when Rothbard was working out his use of a woman's property rights and opposition involuntary servitude to defend abortion, he realized that the rational extension of his logic would have to allow infanticide - at least through exposure and neglect. After all, if a fetus has no right to infringe on a woman's self-ownership by occupying her body, how could he justify requiring a mother to feed, clothe, clean, and otherwise care for an infant after it had been born? If a neglected infant dies, Rothbard asserts, it does so because it cannot justly lay claim to the mother's time, money and effort to care for it.
I reject that position for two reasons. Firstly (and rationally), because both the mother and father were involved in (hopefully) voluntary decisions and choices that directly led to the conception of the child, they must therefore accept responsibility through the implicit agreement thereby entered into for caring for that offspring to some minimum standard, at least until they can find willing parents to take the child. Secondly (and emotionally), my cultural conditioning causes me to reject with horror the idea that an live infant should be subject to fatal neglect without consequence to the responsible party. On the other hand, if I were from another culture that is more tolerant of infanticide I would likely not have that same reaction.
To be fair to Rothbard, he also believed that infanticide wouldn't be a huge problem because in a free society, infants would be a marketable commodity and the parents of unwanted infants would have an incentive to provide for them until it could be sold to parents who do want the baby.
