Jonathan Catalan has a brief post with an extensive comment section that works through the classic critique of democracy and "social contract" ideas more broadly. I concur with a lot of these critiques of the social contract, and I've said as much in the past. I've been meaning to flesh out my thoughts on this in more detail for a while, but I'll mention it briefly here as a sort of response to Jonathan.
1. Human beings are social beings and they are constantly interacting. These interactions are sometimes voluntary and consistent with human liberation, and sometimes involuntary and inconsistent with human liberation. Voluntary and involuntary interactions can both be either private or corporate. In Truth and Power, Foucault talked about an idea that I think is very similar - the "web of power". We are caught in a web of involuntary impositions on our liberty. It is the human condition.
2. There is often recourse for these impositions. Sometimes we have institutions like courts, or we have parents that arbitrate disputes between siblings. We can always revolt or strike back (self-defense is almost universally considered an appropriate justification for violence). But the ability to respond to an imposition doesn't make that imposition voluntary. Recourse to courts does not change the fact that pollution is an assault on personal liberty. Recourse to revolt does not change the fact that a tyranny is an assault on personal liberty. It is very good that we have recourse in many situations, but it doesn't make the involuntary act voluntary.
3. The state is involuntary. A social contract is distinct from other contracts precisely because the parties to the contract don't necessarily agree to their inclusion. However, the state can evolve or be designed (or a little of both... and after all, when it comes to human institutions what is "evolution" and "emergence" except for an accumulation of designs over time?) in a way that deliberately privileges citizen participation, control, and decision making. So yes it is involuntary, but that does not mean it is the equivalent of slavery. We need to be careful not to embrace sentimentalism that confuses the issue in either direction: the state is involuntary, but it is also an institution that is controlled by the people (at least the kind of states I'm concerning myself with here are).
4. The purpose of the liberal state is to minimally impose itself on human liberty but act as a bulwark against private tyrannies that emerge in the "web of power" that is natural to the human condition. The reason why non-libertarian classical liberals still embrace the state is precisely because they are concerned about private tyrannies that would threaten liberty in the absence of the state, and they conclude that in the optimal balance between private tyrannies and the public tyrannies implicit in the social contract, there is a role for the state. This brand of classical liberalism is not libertarian precisely because we consider libertarian restrictions on the state to threaten human liberty. Libertarians act like we don't care about human liberty, but the whole point of why we're not libertarians is because of what we perceive to be the libertarian threat to liberty. Libertarians downplay or ignore private tyrannies for some always unexplained reason. Tyranny is almost always public for them, and discussions always come back to government. Why? I still don't know. But because they see things that way they are always going to have a blind spot and they will always be an inadequate defense of human liberty. I don't really care about the etymology of their label - I'm not a libertarian because of how much I value liberty. There are definitely worse things to be than a libertarian if you value human liberty, of course. Libertarianism is definitely high on the list of second-bests. But it is not ideal.
5. This conception of the liberal state excludes non-liberal states as surely as it excludes libertarian states (and libertarians almost universally embrace the existence of some form of state). I don't have to answer for Nazi Germany or some other tyranny any more than any other non-anarchist, non-Nazi does. Slippery slopes are legitimate to talk about, but (1.) don't ignore the slippery slopes of libertarianism, which slide to anarchy on one side and crony capitalist oligarchy on the other, and (2.) don't think you'll get away with convincing me that the liberal state is on the road to serfdom. Libertarians love to bring up the slippery slope but they hate to apply it to themselves. Whenever crony capitalism emerges it's never the bottom of their own slippery slope. They won't even consider the possibility.
6. I'll also note that my thinking on this has been influenced by John Dewey's political writings as well. There's a piece where he talks about freedom in the same way that Foucault talks about the "web of power". I'll track it down, but he basically points out the path dependence of our ideas of freedom. Often (not always) when people complain about violation of their rights as a result of a state action, that state action has relieved a violation of other people's rights. When people complain about state imposition on BP and violations of liberty, what they forget is that this action helps to relieve impositions on the liberty of people suffering from the spill. Bastiat might call it the "seen and the unseen". Defenders of BP ignore the "unseen" violations of the liberty of the people the state is helping. In other words, we're dealing with a dense web of freedoms and rights, just like Foucault's dense web of power. Often it's a trade-off between competing liberties, and often we accept involuntary state action because it is less pernicious than involuntary private action.
So that's a long sketch, but believe me it's still just an outline. I think you get the gist, though.