Some people don't like either of the two main parties

There is no point having democracy and letting people vote if they want to be slaves. We give people a vote because they want to be free and democracy enables the people to choose which politicians will give them the most freedom. First-past-the-post systems tend to result in a two-party system. This is so that (due to the fact that) voters make sure their vote is not wasted by voting for a politician who has no chance of winning. If we assume that people vote for the politicians which will give them the most freedom (democracy is freedom) then if a voter votes 'tactically' then their freedom is being compromised. (And tactical voting is evidence of freedom being oppressed.) It might be the case that by chance the voting public splits exactly into two categories in which case the two-party model might be suitable. However it is much more likely that the people would be best reflected and represented by a plethora of disparate parties... with plenty of choice for the voter at the ballot box. When voting behaviour is altered due to the voting system then we know that the freedoms of the people are being compromised... since we assume in a democracy that people want freedom. The people are needlessly being oppressed by the voting system if voting for their naive (instinctive) choice without the consideration of tactical voting results in less democratic influence. If they must vote tactically and are not otherwise inclined to support either of the two main parties in a fptp system then they are being oppressed by the system. Only someone who 'likes' one of the main parties is not being oppressed by the fptp system. Fptp is oppressive if (some) people do not like at least one of the main parties. If neither of the main parties is appealing to some voters then fptp will be reducing freedom overall.