The state is unpopular

Without elections and democracy the state has no authority... it is only because the state has a mandate from the people that it is able to claim any legitimacy. But the first-past-the-post (fptp) voting system tends to result in a two-party oligarchy which means the state doesn't have a true mandate. Democracy is a bulwark against the state becoming too powerful but if fptp is used then the people are unprotected. The state has not been given the permission of the people if only fptp has been used to have a true mandate the state must use a proportional system. All valid systems of democracy have the characteristic of proportionality otherwise there will be an incentive to vote tactically due to there being fewer candidates with a chance of success (which reduces choice). Without the use of pr the members of the state (the politicians) have not given the people sufficient opportunity to reject them. The people have not been given a veto if there is not proportional representation and so the state is invalid. There is no (valid) state if a form of proportional representation has not been used.

First past the post is a form of authoritarianism

If a democratic system is inherently monarchist then we cannot say that the system is anarchist. We might imagine that all forms of democracy are anarchist since the people would be unlikely to choose an authority above themselves (at least not knowingly). But if voters have only two (realistic) choices available to them then this is not anarchistic and the voters are subject to a form of authoritarianism even if it only concerns a duopoly and not a more serious monopoly. We are not free if there is some form of authoritarianism above us however benign it might at first appear. We can only be free under a democracy if the system ensures a proportional outcome. If the people are not given a proportional system of democracy then they are not free because there is a (democratic) authority above them.

First past the post is a monarchist system

In a democracy people should be able to vote for whomever they like and have their choice respected... but if a winner-takes-all system is in place then people must consider voting tactically. If proportional representation is not assured then it might make sense for voters to vote tactically if their fear of one of the main parties is greater than their desire to support their preferred party. If the state doesn't make sure to guarantee a proportional outcome then the voting system will suffer from the truth of Duverger's law and it is likely that a two-party system will emerge.

If there is a two-party system then one will tend to be more 'socialist' than the other simply because this is the primary axis of all political conflict. So then the party on the right will be seen as a more monarchist (and perhaps liberal) party and the party of the left will tend toward socialism and communism. If people are not monarchists and reject the ideology of monarchism they are left with only the socialist party with which to register their opposition to monarchism. Such voters might decide that they are willing for their vote to be effectively ignored (by voting for a smaller party). So a vote which under a proportional system would be in opposition to socialism does not register in a two-party system. The winner-takes-all system results in a two-party system which makes it impossible for non-monarchist liberals to reject socialism so then over time (assuming the electorate is not communist) a two-party system will be more socialist than a multi-party proportional system. The people will be more liberal than the two parties in a first-past-the-post system. It is a cost to liberal voters to force or compel them to vote for the monopolist centre-right party and this cost will result in less support for the parties which oppose socialism. There is no advantage in assuming the voters are not liberal (assuming that democracy is communism) because even in doing so there is no advantage to be gained... when we choose between different systems of democracy we must assume that the people are liberal not because they necessarily are liberal but because there is no possible way to 'punish' the voter by reducing their choice. Trying to coerce the voter into voting for a particular party (to justify fptp) has the bad outcome of repelling voters from the democratic process. If we are to have democracy then we must assume that voters are liberal and not communist or fascist (otherwise we are advocating monarchy). First past the post does not prevent against fascism (if the people are fascist at least one of the main parties would be fascist as well) and so there is no reason not to use a proportional system.

It is a crime for the state not to use democracy

The purpose of democracy is to enable the people to liberate themselves from the state. The state is criminal if it seeks to assert itself without the consent of the people... governments which do not seek a democratic mandate are tyrannical and authoritarian. It is a crime for the state not to use democracy. But not all forms of democracy are the same... if the government is elected using a winner-takes-all system this means that voters are rewarded for voting tactically. If voters vote tactically this means they are not able to fully express their political preferences and their liberties have been infringed upon. Not to use the most full form of democracy (proportional representation) is to oppress the people. The only way for the state to avoid being authoritarian and illegitimate is to use the fullest possible form of democracy which is proportional representation. If the state doesn't use this kind of democracy then it can be described as criminal.

There is no good reason to oppress democracy

We know that the state is fallible otherwise there would be no reason to have democracy we would just give a comprehensive mandate to the state. The reason to have democracy is to put a check on the powers of the state because the state (purportedly) has a legal monopoly on the use of force. The state is potentially very dangerous as we have seen in history and it has the power to remove freedoms from the people. So democracy is the means by which the people protect themselves from the state and it is how they retain their freedoms. Democracy is synonymous with freedom... where there is a state. If there is no state then of course democracy means nothing. If democracy is synonymous with freedom and we assume that freedom is good then there is no good reason to oppress democracy. But it is possible to argue that the first-past-the-post (fptp) system is an oppression of democracy since it does not give the voter the opportunity to vote for whomever they would like. The absence of proportionality means that so-called tactical voting is possible and rewarded which means that the typical person has not been afforded their true democratic rights and freedoms. Fptp is an oppression of democracy and so then by extension it is an oppression of freedom. There is no freedom if there is a state which does not respect democracy and so first-past-the-post is not freedom.

First past the post has no virtues

If we have a democracy then we must respect the voter. The purpose of democracy is to give a voice to the people and to make sure the government is responsive to their views. So then it is consistent with democracy to respect the voter but with a first-past-the-post system the voter is not respected (unless they vote for one of the leading parties). To punish the voter for not supporting one of the main parties is to disrespect the voter which is inconsistent with democracy itself. The voters (we must assume) are innocent and good and so then there is no reason to punish them with first-past-the-post. There is no reason to punish innocent people and the voters are innocent and so then there is no reason to use fptp.