The latest television advertisement for the No campaign for the same-sex marriage survey has been deemed not suitable for children because of its explicit content.
The advert features two women who have appeared in earlier television campaigns.
As the women talk about their concerns for children being taught about homosexual intercourse and gender issues, cards come up claiming to quote school resources linked to the controversial Safe Schools programs.
The body responsible for classifying ads, Free TV, determines whether commercials can run and at what times.
"This ad was given an MA rating because of references to sexual activities," acting CEO of Free TV Australia Pamela Longstaff said.
The use of the words penis and vagina in context were "relevant to the classification given", Ms Longstaff said.
"There's nothing special about this ad, it's one of many that are classified," she said.
The ad is in line with Coalition for Marriage No campaign which has linked the legalisation of same-sex marriage to concerns about forced homosexual or transgender education in schools.
'Ironic' ad was not suitable for children: Shelton
No campaign spokesman Lyle Shelton said it was "ironic" that the ad was not deemed suitable for children.
"They said the content was inappropriate for children's viewing time," Mr Shelton said.
"Now that's quite ironic from our point of view because this resource was made available to children in schools from the Victorian and South Australian governments."
Mr Shelton said the organisation had not tried to negotiate changes of the content with Free TV so it could get a lesser classification and the ad would go to air on Monday night.
"There was always the risk that regulator would take this action," he said.
'No schools forced to join'
A Safe Schools Coalition of Australia (SSCA) spokesperson said participation in the Safe Schools was voluntary.
"No schools are forced to join Safe Schools or use any of the resources and schools looking to become formal members need to have parental consent," the spokesperson said.
"The only in-classroom resource SSCA offers is the evidence-based All of Us unit guide which was written by a panel of education experts.
"If teachers decide to use this resource school leadership must obtain consent from the relevant parental body."
The Australian Bureau of Statistics last week estimated 10.8 million Australians had returned their survey or two thirds of those eligible to vote.
