The Liberal Democratic Party senator says anxiety among Coalition MPs about their pre-selections for the 2016 election is affecting the timing of a marriage equality debate in Parliament.

Senator Leyonhjelm has been pressuring Prime Minister Tony Abbott to grant his MPs a free vote on marriage reform.

But he recently delayed plans to introduce a private member's bill to change the marriage act and is still deciding whether to proceed this year or hold off for several months.

It comes after Opposition Leader Bill Shorten used a keynote address to the Australian Christian Lobby on Saturday to confront the conservative lobby's agenda and declare that his support for same-sex marriage was consistent with his Christian values.

"I believe in God and I believe in marriage equality," Mr Shorten said.

Senator Leyonhjelm said granting of a conscience vote would come at "no political cost" to the government, but in the event that did happen MPs who were supportive of marriage equality still feared political penalties for publicly saying so.

"They're a bit scared because there's push-back coming from higher up in the party," he said.

"Some have expressed concerns that they might jeopardise their preselections due to conservatism in the party, then there is also the prospect that there might be a negative consequence to their vote in their electorate."

At the Christian Lobby conference, while delegates disagreed with Mr Shorten's views, they welcomed his openness on the subject.

In a question-and-answer session after Mr Shorten's speech, ACL managing director Lyle Shelton immediately mentioned "the elephant in the room", saying "We really appreciate the way you've been fearless and frank with us".

Senator Leyonhjelm does not want his bill to be debated until he knows he has a majority in both houses of Parliament to pass the historic reform.

The Australian Marriage Equality group and a Canberra-based family law firm are hosting a private screening of the documentary The Case Against 8   in Parliament this week in a bid to convince government MPs in particular "they can be brave" and support marriage equality.

Ted Olson,  the Republican lawyer who fought on the 2009 case challenging Proposition 8, a constitutional amendment in California that banned same-sex marriage, will speak at the screening.

He told Fairfax Media marriage equality was a conservative value and conservative politicians should be speaking about it.

"That's what leadership is all about," Mr Olson said.

Public opinion had changed rapidly and conservatives who did not support reform risked not only alienating same-sex couples but their supporters, he said.

"Conservatives will continue to be a diminishing part of our political culture so long as they don't respect the dignity and rights of same-sex couples.

"If you believe that two people who love each other should be treated with dignity, they should have equality and respect, they should have the same liberties and happiness, then you should support same-sex marriage.

"Conservatives should be speaking out about it," Mr Olson said.