Two senior Coalition MPs have suggested that Australia could do away with the Marriage Act entirely and instead have civil unions for everyone, with only religious organisations able to marry couples.

Veteran Liberal MP Philip Ruddock, who backs the traditional definition of marriage, made the suggestion on the ABC on Monday night, referring to France's approach where all couples must get married in a town hall ceremony. Religious organisations then have the option of conducting their own ceremonies.

The government's new envoy for citizenship told Fairfax Media on Tuesday that the French model was just something he had read about "over time".

"And will continue to note it as an alternative," he said.

Mr Ruddock said the purpose of raising the example was to make clear that "there are a range of options that are worthy of consideration".

When it was noted this would mean that non-religious couples of any sexuality would not be able to "marry," Mr Ruddock said, "It may be an issue that if they want to have a marriage that they want to go to a church or a synagogue".

His suggestion was reinforced by conservative Coalition member, Social Services Minister Scott Morrison, who criticised "the presumption that there is only one answer" to the same-sex marriage debate, before referring to Mr Ruddock's comments.

"So there would no longer be, under that proposal, a Marriage Act as such," Mr Morrison told 2GB radio.

"There would be a civil union act where you would get equality of all relationships before the law and then issues of marriage would be determined by the various religious bodies."

The idea was slammed as "appalling" by marriage equality advocates.

Australian Marriage Equality national director Rodney Croome said Mr Ruddock and Mr Morrison had misconstrued the French approach, where the civil ceremony still results in a marriage.

"Philip Ruddock and Scott Morrison seem to be proposing the complete abolition of legal marriage, which is definitely not what has happened in France, or any other country for that matter," he said.

"I find it appalling that defenders of traditional marriage would prefer to wreck the institution than allow same-sex couples to marry.

"The millions of Australians who are happily married would also find it astounding that conservative politicians are proposing something as radical as taking their legal status away."

Australian Christian Lobby managing director Lyle Shelton insisted the current arrangements for marriage should be retained. But when pressed, he said Mr Ruddock's suggestion was an option if the "worst" happened and same-sex marriage was legalised.

"I think it's a hypothetical but it's something that could be considered if the worst happened," he said.

Mr Shelton said the main reason for denying gay people the right to marry was to maintain Australia's ban on commercial surrogacy. The Australian Christian Lobby is concerned that children would be denied the right to know their biological parents.

"Going to some sort of two-tiered system then you haven't resolved that issue of how a child is allowed to know who its mother and father is," he said.

Amid some Coalition MPs warning that the Prime Minister could face a backlash if same-sex marriage is legalised, the Christian Lobby joined with 38 other religious leaders to write to Tony Abbott urging him to uphold the existing definition of marriage.

"To uphold marriage as the mutual love of a man and a woman, open to the gift of children, is not bigotry or prejudice," it says. The signatories of the letter include the Anglican Diocese of Sydney, the Syrian Catholic Church and the Seventh-day Adventist Church.

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