Fptp protects the mainstream parties from libertarian voters

We can assume that if there is a centre-right majority (among the electorate) Fptp does not prevent it from gaining power. So then if the mainstream centre-right party allows freedom (is libertarian) then there is no problem with Fptp voting. However, if the centre-right party is not libertarian then Fptp presents a problem since an authoritarian party will be returned in either case, and we cannot differentiate (at the ballot box) which type of right-wing party we would like. Fair votes would enable voters to choose which type of right-wing party they would prefer.