Prime Minister Malcolm Turnbull is facing fresh political pressure over the issue of same-sex marriage. On the one side, members of his own party claim they won’t be bound by the results of a plebiscite, or people’s vote, on whether same-sex marriage should legalised.

Opposition Leader Bill Shorten, meanwhile, has called on Turnbull to dump the plebiscite and instead allow a parliamentary conscience vote.

The Liberal Party leadership claim that a plebiscite should be embraced because it is a wonderful manifestation of democracy. They have even ridiculed Labor for suggesting that parliament, rather than the people, should decide the issue.

The brand of democracy supported by Liberal Party members is liberal democracy. It would be strange if this were otherwise given the party’s name. Nevertheless, if members of the Liberal Party really do support liberal democracy, they should oppose holding a plebiscite. Here’s why.

How would you like it?

A plebiscite demonstrates quite clearly that some couples are being treated as less-than-equal citizens. A vote on whether they can get married is discriminatory because it applies a standard to them that does not apply to heterosexual couples.

This becomes apparent if we turn the tables on those arguing for a plebiscite. I ask every heterosexual member of the Liberal Party (and reader of this piece) to answer the following:

Do you think it would be appropriate for a vote to take place on whether you can get married? Do you think that the permissibility of opposite-sex marriage should be determined by a popular vote?

My guess is that most people will give a resounding “no” to these questions and that readers would find it demeaning, insulting and exclusionary if their right to get married was determined in such a fashion.

So, unless there is some compelling normative argument for why Australia should have a plebiscite on same-sex rather than opposite-sex marriage, I suggest we should treat both cases in the same way.

One response might be that heterosexual people already have the right to get married, so we don’t need a popular vote on the issue. This response simply states a fact and does not get to the issue of whether it is right or wrong for such a vote to take place.

Even given this fact, we do not need a plebiscite. All we need is for MPs to recognise that a wrong can be righted by changing the law. It is necessary for a vote to take place in parliament, but not in the country at large. Instead, the Liberal Party wants to spend more than A$150 million on an act of discrimination.

Liberal democracy has its limits

What the Liberal Party does not seem to recognise is that the philosophy of liberal democracy puts limits on what can and cannot be decided by democratic means. In particular, it starts from the assumption that certain rights and liberties are beyond the reach of democratic decision-making.

In other words, the “liberal” part of the idea puts limits on the “democratic” part. Sometimes these limits are made explicit and reified in constitutions. Sometimes they are not. But all liberal democracies are based on this assumption.

Liberal democracies, therefore, are not neutral political zones. They are embedded with values that rule some things in and, most importantly, rule some things out as appropriate areas for government intervention.

Good arguments can be made on liberal democratic grounds for why government should, or should not, get involved with marriage. But if it is going to get involved it should do so in an impartial manner that respects all citizens.

The reason for this impartiality is that citizens of liberal democracies are meant to be treated equally. As political philosopher Joshua Cohen writes:

… the background conception of citizens as equals sets limits on permissible reasons that can figure within the deliberative process.

It has taken a long time for liberal democracies to recognise that their philosophy and their policy has been out of step on same-sex marriage. They have started to address the imbalance. Britain and the US did so without resorting to a popular vote; Australia’s parliamentarians could easily have followed their lead.

We should also remember that the only reason there was a referendum in Ireland was because its Constitution had to be changed. Australia has no such requirement.

Turnbull often trumpets his belief in liberal democracy. He can put his values into practice by supporting a call for all members of his party to vote in favour of legalising same-sex marriage. While this won’t happen, it is the best option open to him if he takes the philosophy of liberal democracy seriously.