Opponents of the referendum on same-sex marriage say voters should recognise the special role of mothers and fathers.
The group 'Mothers & Fathers Matter' said voting Yes would mean the state is unable to recognise the benefits of children being raised by their biological parents.
It said that the law should recognise equality between all people, but that not all relationships are the same.
The group's chairman chairman Professor Ray Kinsella said: "I do come back to children…We believe in respect for all loving relationships. We simply say - they're not all the same."