The speaker was not a same-sex marriage advocate but Lyle Shelton, the face of the "no" campaign and managing director of the Australian Christian Lobby.

Mr Shelton, outlining his case for the status quo at the National Press Club, began by apologising for Christians' treatment of gay men during the AIDS crisis of the 1980s.

"People who represented Christian organisations said some very hateful and hurtful things," he said, and conceded there was vitriol "on both sides" of the contemporary debate over same-sex marriage.

But there was no apology for Mr Shelton's own 2013 remark that same-sex marriage would "create another" Stolen Generation by taking children away from their biological parents.

Mr Shelton said he was confident none of his comments breached anti-vilification laws - including the "so-called safeguards" rushed through Parliament on Wednesday that impose civil penalties for vilification, intimidation and threatening behaviour during the campaign.

He appeared alongside Liberal Party federal vice-president Karina Okotel before an enthusiastic audience that included Coalition MPs Matt Canavan and Eric Abetz.

Mr Shelton rejected charges of homophobia, arguing the concept was not prevalent in Australia on a strict definition of the term. Instead it was a "convenient slur" used to tar opponents of same-sex marriage, he said.

And in a reflective moment, Mr Shelton lamented the decline of marriage as an institution, which he blamed not on the LGBTI movement but straight people.

"We have a big job to do as a society to rebuild a marriage culture," he said. "It is not gay people, or the same-sex marriage movement, that have trashed marriage in this country. We have done a pretty good job as heterosexuals over the last 50 years."

Mr Shelton was less forthcoming about the budget of the Coalition for Marriage, which is supported by the ACL, major churches and a collection of other groups against same-sex marriage.

Both the "yes" and "no" camps have asserted they will be outspent by their opponents but have refused to divulge their budgets or expenditure.

It was revealed on Wednesday the "yes" campaign received a $1 million personal donation from Qantas chief executive Alan Joyce. Mr Shelton said the "no" camp had not received any funding from evangelical churches in the US.

Ms Okotel declared she had once supported same-sex marriage but flipped because of concerns over "consequences", such as increasing sex-and-gender education in schools.

She raised the example of a US school in which children were shown a picture book depicting a same-sex couple kissing. Challenged on this, she later said pictures of any two people kissing was "sexualised content" that should not be shown to children.

"I wouldn't be happy with my children in the early years of primary school being read books with people kissing in it," she said.