A PREGNANT woman and her partner rapt with the news "You're having a lesbian" are the newest stars of an Australian campaign to promote gay marriage.

The controversial ad shows the young couple at an ultrasound appointment. Nodding their heads when asked if they want to know what they're having, they break into emotional smiles at the news that they are "having a lesbian".

Similarly, the campaign's billboard pictures the pregnant woman smiling and baring her belly, with the words "Congratulations, you're having a lesbian."

New Zealand MPs passed the Marriage Amendment Bill allowing same-sex and transgender couples to marry in April.

The theme of the campaign is "Any child can be born gay. So marriage equality is every family's issue."

Parents and Friends of Lesbians and Gays spokeswoman Shelley Argent told news.com.au the provocative clip was angled at mainstream families, and based on the understanding that gay children were "born that way".

"Being lesbian or gay is not a choice," she said.

"It's got nothing to do with culture or parenting skills. It's just the way you are. You're born that way."

Campaign creator Steve Minon, a Brisbane freelancer, said he designed the ad to "get people thinking, and challenge them a little".

"We're trying to point out to any family that marriage equality can be your number one priority, without your knowing about it."

Queensland billboard giant GOA has donated $60,000 of billboard space for the ads and company CEO Chris Tyquin said he will be asking interstate billboard and signage companies to donate space as well.