THE gay marriage debate had been "conclusively won" and the matter was "settled", a Liberal frontbencher told a Christian lobby group today.

Opposition families spokesman Kevin Andrews addressed the Australian Christian Lobby's national conference in Canberra today.

He told the gathering that if the Coalition had granted its MPs a conscience vote in the gay marriage debate it would have made no difference to the outcome.

Outside the conference at the Hyatt Hotel about 20 marriage equality campaigners protested with banners and chanted "hey hey ho ho these homophobes have got to go," as a sole policeman watched on.

Mr Andrews told the conference the battle to protect marriage in the federal parliament "was conclusively won". He said most MPs will now take the view that the matter is settled for some period time.

"The battle will now move to the states," he said, adding that he believed state governments did not have the constitutional power to make changes to marriage laws.

He said there would likely be High Court challenges against any state legalisation of gay marriage.

It would be an "extremely adventurous High Court" to fly in the face of the Commonwealth parliament twice in the last decade deciding to keep the status quo.

During the gay marriage debate in federal parliament in September, both houses rejected private members bills to legalise same sex marriage.

Labor allowed its MPs a conscience vote but the opposition did not.

One Liberal senator, Sue Boyce, spoke in favour of legalising gay marriage but she abstained from the vote.

Former leader Malcolm Turnbull had said he would have voted in favour of gay marriage had a conscience vote been allowed on his side of parliament.

Mr Andrews said the coalition had crunched the numbers on the hypothetical prospect of a conscience vote.

"You don't last in politics for 21 years unless you can count," he said.

"(We) counted the numbers ...

"The reality is it would not have made much difference what so ever to the numbers.

"There would have been half a dozen people... who would have voted the other way."

Mr Andrews made a veiled swipe at Prime Minister Julia Gillard for pulling out of addressing the Christian Lobby's conference.

Ms Gillard cancelled the speaking engagement after the Christian Lobby's head Jim Wallace was widely criticised for saying smoking was healthier than homosexual lifestyles.

Australian Marriage Equality spokesman Rodney Croome said Mr Andrews' predictions were "just wishful thinking".

"Mr Andrews is mistaking a lull in the storm for the end of the storm," he said.