Progressive Christians say a TV advertisement opposing same-sex marriage is "fear-mongering", "irresponsible" and "wrong".
Australian Christians for Marriage Equality on Wednesday launched a national campaign encouraging people to vote Yes in the federal government's planned postal survey on gay marriage.
They said a Coalition for Marriage ad released overnight didn't represent the views of most Christians.
"To put out an ad that links same-sex marriage with the safe schools program is ... an admission of failure," Anglican Reverend Dr Keith Mascord told reporters in Sydney.
"It's not looking at the substance of the debate but fear-mongering. It's irresponsible and wrong and we abhor it.
"God wants all people to thrive and that's what we want and that's why we support marriage equality."
The TV ad features three mothers highlighting their concerns about how same-sex marriage would affect what is taught and promoted in schools.
One mother says "the school told my son he could wear a dress next year if he felt like it".
The Anglican Dean of Brisbane, Dr Peter Catt, says religious freedom and gender identity are issues that are muddying what should be a simple debate.
"Gender debates in schools and all those other issues are simply being introduced to muddy the water," he told reporters in Brisbane.
"This is a very simple concept that we extend the Marriage Act so that it applies equally to all persons."
Australian Christians for Marriage Equality representative Benjamin Oh said the ad was hurtful to those who identified as part of the LGBTI community.
"It is very sad that kind of messaging is going out, I know it hurts families, I come from a devout Catholic family ... and I know they hurt with me," he told reporters on Wednesday.
"They are powerful people with lots of money to spend on these big flashy advertisements, we don't have that, but we have the people with us, the people of God, Christians voting Yes."