Opponents of same-sex marriage are officially launching a national campaign as millions of Australians cast their votes in a postal survey.
The Coalition for Marriage will hear from speakers including senators Cory Bernardi and Matt Canavan and Australian Christian Lobby chief Lyle Shelton in Sydney on Saturday night.
Spokeswoman Sophie York said the group is concerned about the possible flow-on effects of changing the Marriage Act.
"Many parents are concerned about their rights as radical gay sex and gender education classes become more widespread and even compulsory," Ms York said.
"Changing our marriage laws also gives license to activists who will weaponise anti-discrimination laws to target those who hold to the current definition of marriage."
The launch comes after an estimated 30,000 people rallied in Sydney on Sunday in support of same-sex marriage.
Prime Minister Malcolm Turnbull and Opposition Leader Bill Shorten have both assured Australians any legislation making same-sex marriage legal will come with religious protections.
The leaders have long declared their support for marriage equality.
The Australian Bureau of Statistics is aiming to get the survey forms out to 16 million voters by September 25, with almost half already landing in letterboxes across the country.