Under pr voters would be able to leave an unpleasant party

The advantage of proportional representation over Fptp is that voters are able to endorse only a narrow form of ‘aggression’ by the State which they approve of, it is easy to reject an excess of aggression. With Fptp both of the leading parties may be aggressive in ways we do not like but we must choose one of them, if we vote. If we assume people will naturally seek to minimise the aggression they perpetrate at the ballot box, pr will led to a more liberal society, for this reason. If a particular aggression is insisted upon by more than half of the voters then it is likely that a law will result. Under Fptp it is in the electoral interests of the parties to offer a broad range of aggression so that votes will not be lost. Swing voters (like any other voter) may be unreasonably prejudiced in a particular way, consequently, both of the leading parties will need to promise a certain type of aggression which would not be relevant under pr. Normally, with pr this would be offensive to the rest of the voters (and hence damaging to the party) but with Fptp they have nowhere else to go and will vote for them anyway. Being unpleasant doesn’t sufficiently hurt the party for this reason, voters are not able to (or at least generally they will not) desert and punish their chosen party if it is aggressive in ways which they do not like, voters are stuck and cannot leave.