If political philosophy is a branch of moral philosophy (and it is), then to have a political philosophy assumes having–even if only in an inchoate form–a moral philosophy.
If that’s true, then the claim (and it’s a rather common one) that libertarians should only concern themselves with permissible state action and take no stand on relationships and power structures outside of the state’s control is, I fear, rather incoherent. Because being a “thin libertarian”–as this view is called, in opposition to “thick libertarianism”–means (1) having a moral theory justifying your libertarianism but (2) believing that moral theory doesn’t also have something to say about relationships and behaviors outside of (the proper sphere of) politics.
Put another way, anyone who claims to be a libertarian (or claims any other political philosophy, for that matter) is a libertarian because she holds certain moral views about how people should–or are permitted to–interact with each other. Views such as, “Initiating aggression is always wrong” or “People have equal moral worth, and so should be treated equally or given equal say.”
Those moral beliefs then lead the libertarian to hold certain politicalbeliefs about the legitimate role of the state–or, for some, beliefs about the state’s inherent illigitimacy. But if those moral beliefs are strong enough to motivate a political philosophy, they also must be strong enough to lead to conclusions about human interactionoutside of the political sphere.
This means that anyone who is libertarian because of foundational moral beliefs (which is most of us who have thought deeply about our political views), must be a thick libertarian–even though they (likely) believe that the state should not enforce many (or most) of the conclusions their moral philosophy leads to. Because the very nature of a moral belief is that, if we believe something to be a moral truth, then we believe people ought to follow it. And if we believe people ought to do something, then we ought to want them to follow it. Or, at least, think the world would be better if they did follow it.
Of course, just because two different moral philosophies may both lead to libertarianism, it doesn’t follow that their non-political views (the “thick” part of their “thick libertarianism”) will be identical, or even compatible. That’s okay! But that pluralism should not lead us to think that libertarians should remain silent about moral questions outside of the (proper) realm of politics.