The Australian Christian Lobby has expressed disappointment that ordinary Australians are being shut out from having a say about the biggest social policy change in a generation.

Labor's decision to block the people's vote on redefining marriage shows that political elites in Labor, the Greens, the Nick Xenophon Team and Senator Derryn Hinch do not trust the public to have a respectful national conversation.

Australian Christian Lobby Managing Director Lyle Shelton said it was only three years ago that Opposition Leader Bill Shorten was supporting a plebiscite.

"During the 2013 election campaign Mr Shorten told an ACL candidates’ forum that he supported a plebiscite.”

Mr Shelton said same sex marriage activists had always found it easier to intimidate parliamentarians with the threat of being labelled ‘homophobes’ or ‘bigots’ if they did not fall into line.

"Despite constantly claiming overwhelming public support for redefining marriage, it is perplexing that advocates are not willing to trust the Australian people with a decision that has enormous consequences for all families.

"Such a big change with such big consequences for what children are taught at school, the rights of children to know their mother and father and for freedom of speech should be taken to the people," Mr Shelton said.

"The Coalition went to the last election promising a people's vote and won the election.

“It would be a breach of trust by government members to allow any other pathway for change.”

Mr Shelton said ACL would redouble its efforts with marriage coalition partners in coming months and years to ensure the Australian people understood the consequences of change.

“There is no discrimination in Australian law against same-sex couples but there are significant consequences if the definition of marriage is changed.”

Mr Shelton said many Australians would be disappointed at a lost opportunity to resolve what has been a long running debate.