The group is claiming that same-sex marriage will increase suicide because people who are against it will be bullied over their views if it becomes law.
The group posted a campaign image on social media saying “same sex marriage increases PC bullying in the workplace”, with a picture of a sad-looking woman with a rainbow-coloured noose around her neck.
Mental health advocacy group beyondblue tweeted to the group and condemned them for their “harmful” message.
“@MarriageAll we’re advocating strongly in this space to reduce stigma & harmful messages around suicide like this,” their tweet said.
“You may not have thought about it, but you may want to consider the harm that depicting someone in a noose may cause.”
Social media has erupted with thousands of angry comments, many calling the group out for their lack of evidence.
“This is sick. You can’t come up with a real argument to support your nasty hate so you resort to this hateful fantasy. Shame,” David Fowles wrote.
The Marriage Alliance was unapologetic.
@beyondblue a biased position in the same sex marriage debate deflects from core mission. A neutral stance is always best governance.
— Marriage Alliance (@MarriageAll) February 3, 2016
* Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14 or Suicide Call Back Service on 1300 659 467.