The Coalition has decided against a free vote on same-sex marriage, but on Tuesday night Tony Abbott announced that a national vote could be on the cards as his party worked out its 2016 election policies.

If there was to be same-sex marriage, "the disposition ... is that it should happen through a people's vote rather than simply through a Parliament's vote", Abbott said.

It sounds simple, right? Let the people vote. Let's decide this thing, once and for all.

While it is true that the Australian Christian Lobby has been calling for a plebiscite for some months, marriage equality advocates have not wanted one, and neither have either the pro or anti camps in Parliament (until yesterday). Legal experts have also dismissed the idea.

Here's why.

1. Cost

While a national vote is not going to threaten Australia's AAA credit rating, they do not come cheap. After all, you need to organise the entire voting population to turn up on a certain day at a certain time.

There is no official costing for a vote on same-sex marriage, but the cost of the most recent national vote - the 1999 referendum - was $66.8 million.

As for doubling up with the next election, to save political energy and taxpayer dollars, Abbott has already dismissed this.

On Wednesday, he declared the next election will be about "a contest between two political movements. I think we can have a separate discussion about where we should go with this important subject."

2. Time

There is a whole bunch of processes to get through in order to hold a plebiscite.

First up, a bill proposing a plebiscite and setting out how it will work needs to be passed by Parliament (expect this not to be a simple ordeal).

This then goes to the Australian Electoral Commission, which organises the mass vote.

Then - because the plebiscite has nothing binding about it - the result needs to go back to the Parliament to work out a bill and then legislate an outcome.

After years of already intense debate on this subject, a plebiscite would kick the issue down the road.

3. Confusion and distraction

Australia already has one referendum on its books - to recognise Indigenous Australians in the constitution - which is due in 2017. And it has a federal election, expected in 2016.

So a plebiscite on same-sex marriage would mean three national polls in as many years or so.

Despite the separate votes, there is no way this will not be a major part of the election debate.

Not only because the Coalition will be deciding its policy before the 2016 election, but because it will set itself up in contrast to Labor, which has already tried to legislate the reform in this Parliament.

Add into this mix what is sure to be a noodle-bending discussion about process. (How and when should a plebiscite be held? What should the question be? Should it be a referendum instead?).

4. Divisive

A national vote on same-sex marriage has the potential to turn ugly. The fact that everything will be on the line for both sides of the debate means big resources and big emotions.

After the Irish result, constitutional law professor George Williams wrote: "In putting a yes/no proposition to the community, referendums necessarily polarise debate. As a result, even if the referendum did succeed, it may leave bitterness and division in its wake.

"This is one of the fears emerging now in Ireland, where sharp opposition to the referendum has emerged from a number of quarters, including in particular the Catholic Church."

5. Not necessary

There is no mention of marriage in the constitution, so a referendum is not needed. And the High Court (when knocking back the ACT government's attempt to bring in same-sex marriage in 2013) has found that the Parliament has the capacity to change the definition of marriage to include same-sex couples.

Australia has only had three plebiscites in its history: two on conscription during World War I and one on the national anthem in the 1970s.

There's a reason for this. As Williams has said: "Plebiscites are rare in Australia. They go against the grain of a system in which we elect parliamentarians to make decision on our behalf."

*A brief word on definitions:

A referendum is a vote to change the constitution.

A plebiscite is a vote on an issue of national importance that does not change the constitution. While they need to be authorised by Parliament, they do not force a government to act.