Australian lobby group Coalition for Marriage has been accused of fear-mongering, after its latest ad suggested same-sex marriage could be used to push “radical gender ideas” on children.

Quoting from a book titled The Gender Fairy , the new “You can say no” ad attempts to draw links between the same-sex marriage postal vote and sex education.

“Only you know whether you are a boy or a girl. No one can tell you,” a quote reads.

Two mothers who appear in the ad also featured in the Coalition’s first ad .

“School programs have no place teaching my son radical gender ideas that he might not be who he was born as,” one concerned mother says in the ad.

“How am I supposed to protect my kids in the future from this stuff?” another adds.

The ad, posted to Facebook yesterday, has drawn backlash.

“I actually do not see the connection between this (inaccurate argument) and marriage equality?” one Facebook user wrote.

“Same-sex marriage and Safe Schools are two totally different things,” another said.

“Amazing how these people try to work on fear and bigotry! If you don't want your children to be taught tolerance (which you obviously don't have) then home school them,” a third suggested.

It comes more than two weeks after the Coalition for Marriage aired its first ad, which also suggested same-sex marriage poses a threat to children.

The ad sparked controversy for suggesting problems would arise in schools if same-sex marriage legislation is passed.

In a statement on its website, the Coalition for Marriage claims “removing gender from our marriage laws means removing gender from the classroom”.

Opposition leader Bill Shorten labelled the ad as "total rubbish", telling Fairfax it was "offensive and hurtful to LGBTI Australians and their families".

Yesterday Prime Minister Malcolm Turnbull supported a decision by a Victorian minister to turn away a heterosexual couple because of their support for same-sex marriage, saying churches are entitled to marry whomever they please.

Opponents of same-sex marriage will today launch 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 tonight.

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.