Same-sex marriage faces uncertain future

It's unclear what will happen now with the same-sex marriage issue, but there are a few possibilities.

WHERE TO NEXT FOR SAME-SEX MARRIAGE?

* PLEBISCITE - The government is pushing ahead with its enabling bill, but the Senate will almost certainly reject it in the wake of the Labor caucus decision to oppose it. It would require an unlikely change of heart by the Greens or crossbenchers to pass the bill and allow the February 11 vote to go ahead.

* FREE VOTE - Malcolm Turnbull has not explicitly ruled out a free vote among Liberal MPs and senators on a bill to change the Marriage Act. But it would be in breach of an election promise and coalition conservatives would revolt.

* PRIVATE MEMBER'S BILLS - There are two bills in the lower house and one in the Senate. But coalition support would be needed to bring them on for a vote, which is rarely given for any private member's bills.

* ELECTION 2019 - Labor has pledged to legislate within the first 100 days of government. It is unclear what policy the Liberals would take to the next election. Public opinion will be critical.

* STATES - A state government could choose to go it alone, but it's likely the High Court would rule it inconsistent with commonwealth law and therefore inoperative.