Without democracy there can be no property

To have property requires democracy because without the right to vote you have no ability to protect yourself from the government. And since property is a natural right then democracy too is a natural right. Democracy is a natural right because property is a natural right and without democracy there can be no property.

For the government not to be arbitrary requires that it seeks a mandate from the people, via an election and this is democracy. If the state doesn't enable the people to hold it to account then it is not a valid government and it is a tyranny. There is no valid government which doesn't use democracy. But we might ask if all types of democracy and election are the same... if voters cannot cast their votes in a 'naive' fashion then they might be encouraged to vote tactically which is a problem for the voters and is not full democracy. Only full democracy validates the state and we cannot describe first-past-the-post as full democracy. To have full democracy requires proportional representation otherwise the voter is not able to have their intentions reflected in parliament.

As an aside we note that since referendums are by nature proportional (each vote is equal) then direct democracy (in the form of government by referendums) falls into the category of proportionally-elected government.

Government is not valid if it is not democracy and if the government is not valid we can say that it is illegal, since the government must be lawful as it is the solitary body which defines the law. If the government is invalid it is illegal and so then any government which doesn't use pr is illegal and we can say that (government by) fptp is illegal. If the state is not held accountable proportionally then it is illegal and fptp doesn't provide legal constraint on the government. The government is insufficiently constrained if a form of direct democracy or proportional representation is not used.