The organisation is also calling for a constitutional guarantee for children to have access to both biological parents
A new group today launched its campaign for a no vote in the upcoming marriage referendum.
First Families First says its concerned that the referendum as proposed would - "amount to an act of vandalism against the constitution".
The group argues that the amendment as it is currently phrased would undermine the biological connections between parents and their children.
Gerry Fahey is one of the three members of First Families First, alongside journalist John Waters and former MEP Kathy Sinnott.
He says there are differing views within the group on same sex marriage but they are against the referendum as it currently stands:
Founder John Waters says the text being put to a vote is an act of "constitutional vandalism" - and has put forward a separate amendment which it says would support the rights of children.
And he says he doesn't think Ireland will vote Yes when it realises the true impact of that vote: