The 43-second TV ad aired on Channel 7 and 9 on Saturday while the parade was under way and shows a mother sitting at a playground table with her young daughter while her husband and son play on a slide in the background.

"We hear a lot about marriage equality, but what about equality for kids?" the woman says.

The advertisement also features David van Gend, the president of the Australian Marriage Forum and a family doctor, who is described on-screen only as a "family doctor".

"So-called marriage equality forces a child to miss out on a mother or a father," Dr van Gend says. "That's not equality for the kids who miss out. That's not marriage."

The ad was part of the Forum's campaign opposing same-sex marriage, called "Think of the Child".

Dr van Gend said the ad was booked and paid for but he received an email from SBS on Friday saying they had pulled it.

"Our review board has instructed that SBS has the right to choose what ads we run, and I've unfortunately been instructed to advise you that we choose not to run this TVC for the Marriage Forum during the Mardi Gras telecast," the email from SBS sales manager for Queensland, Nick Belof, said.

An SBS spokesperson told Fairfax Media that it reserved the right "to determine what advertisements it broadcasts".

Dr van Gend said the pulling of the ad was a "suppression of free speech".

"It is outrageous for a taxpayer funded broadcaster like SBS to apply censorship to one side of the debate on same-sex marriage," he said.

"SBS gives free airtime for them to make their political point on 'marriage equality', but refuses to show even one minute of a paid ad presenting an opposing view."

The advertisement had received regulatory approval as a political advertisement in February, the Forum said.

Dr van Gend has asked the advertising agency the Forum was using to obtain a further explanation from SBS. The agency has been told SBS will give a further explanation next week.

Not-for-profit organisation Australian Marriage Equality's national director, Rodney Croome, said the ad was "actually harming the many Australian children being raised by same-sex couples because it defends discrimination against their families".

"I challenge Croome as to who is "actually harming" children here. A large peer-reviewed study published in January drawn from over 200,000 children, found that emotional problems in children are four times higher in same-sex households than in households with both biological parents. And the difference had nothing to do with social "stigmatisation," Dr van Gend said.

The Australian Marriage Forum's website was registered by the Australian Christian Lobby in 2011.

 

The advertisement triggered a social media backlash, with a Change.org petition set up to remove the ad from television.

@9NewsSyd Australian Marriage Forum ad shown tonight is bigoted scare mongering rubbish! Very disappointed this trash was aired on nine! — Melissa Power (@MelissaPower6) March 7, 2015

Australian Marriage Forum airs out of touch ad, driven by irrational homophobic prejudice. The Ad insults far more than the gay community. — Scott Norton Taylor (@norton_scott) March 8, 2015

At the same time, the Christian Democratic Party was using social media to run an anti-Mardi Gras advertisement featuring a photo of a heterosexual couple and a baby with the caption: "Parenting, not promiscuity is worth celebrating."

Christian Democratic Party leader the Reverend Fred Nile posted his party's ad on Twitter on Saturday, with the hashtag "Mardi Gras 2015".

Parenting, not promiscuity is worth celebrating. #teamnile #MardiGras2015 pic.twitter.com/p04pia1BHV — Fred Nile (@frednile) March 7, 2015

Independent Sydney MP Alex Greenwich, a vocal supporter of same-sex marriage, said: "Same-sex couples make excellent parents, and the children of same-sex couples deserve the right to have married parents.

"I would like to dismiss the ads as being on the fringe of public opinion and clearly not representative of the way in which a strong majority of Australians support the LGBTI [Lesbian, Gay, Bisexual, Transgender and Intersex] community, but I think we also can't underestimate the damage that these ads can do to vulnerable LGBTI people."

NSW Labor Opposition Leader Luke Foley, who has previously voted against same-sex marriage, told Fairfax last month that he had "an open mind" on the issue and would "continue to reflect" on it.

NSW Premier Mike Baird has said he is personally opposed to same-sex marriage but supports a conscience vote on the issue.