Ms Forster's comments on ABC's Q&A come as conservative Coalition MPs warn that same-sex marriage is not inevitable and the Prime Minister faces an fierce internal backlash over what some see as his tacit approval for change.

The City of Sydney Councillor - has also defended same-sex couples' ability to raise children, arguing the most important thing for children is that they are in "loving, supportive, families".

Ms Forster, who is a mother and engaged to her partner, Virginia Edwards, was confronted by a woman in the audience of Q&A on Monday night, who argued that "children are going to miss out" if same-sex marriage is legalised.

"Nature is a mother and a father," the woman in the audience said. "Nothing can change that."

Ms Forster replied that "obviously you cannot have a child without a biological mother and a biological father," before noting that she has four children herself.

"The important thing I think about children and families it that they are in loving, supportive families," she said.

"With all due respect, I don't think that is the exclusive preserve of the traditional family unit.

"I think that children can be loved and supported and brought to their fullest potential with the most wonderful self-esteem, self-confidence and ability to make the most of themselves in same-sex families and in single parent families," she said.

Ms Forster was also asked if Mr Abbott - who is a long-term defender of the existing definition of marriage - had "crossed some kind of Rubicon" on the issue.

"No I don't [think so]," she replied.

"He has a very clearly stated and strongly held view that [marriage is between a man and a woman], I respectfully disagree with him."

Momentum has been building within the Coalition for a free vote, with Queensland MP Warren Entsch and others within the party talking about the Liberals changing their stance and backing a vote in the second half of the year. Mr Abbott has also told Parliament that if the issue is to be dealt with, it should be presented as a cross-parliamentary bill.

Senior Coalition MP Philip Ruddock raised the possibility of a compromise when asked about his view of same-sex marriage on Monday night.

While he said that he had supported the "traditional position" over a long period of time, he also suggested a French-style approach "may be worth a look at".

Mr Ruddock, who Mr Abbott recently appointed as his special envoy for citizenship, said this would involve the government only dealing with the registration of relationships, while the churches would deal with "marriage as we understood it".

Ms Forster said this would not be a satisfactory outcome, arguing that it only required a simple change to the Marriage Act to allow same-sex couples to marry.