The Australian Catholic Bishops have issued a pastoral letter, in which they say that "redefining marriage in the way now proposed would see marriage reduced to a committed, affectionate sexual relationship between any two people."
"In the context of the Irish referendum, both the Labor Party and the Greens have announced they will introduce draft legislation to allow two people of the same sex to marry," the letter says.
"Marriage is both a personal relationship between a man and a woman, and the protective institution for their children. Marriage includes an emotional union, but it goes further than that. It involves a comprehensive bodily and spiritual union of a man and a woman.
"This union of a man and woman is the natural reproductive and protective environment for raising children. Marriage is the foundation of the family unit, which is in turn the first cell of society.
"If the union of a man and a woman is different – not the same - as other unions, then justice demands that we treat that union accordingly. If marriage is an institution designed to support people of the opposite sex to be faithful to each other and to the children of their union it is not discrimination to reserve it to them.
"The Christian tradition teaches that every human being is a unique and irreplaceable person, created in the image of God and loved by Him. Because of this, every man, woman and child has great dignity and worth which can never be taken away. This includes those who experience same-sex attraction. They must be treated with respect, sensitivity and love.
"Redefining marriage in the way now proposed would see marriage reduced to a committed, affectionate sexual relationship between any two people. All marriages would come to be defined by intensity of emotion rather than a union founded on sexual complementarity and potential fertility. Husbands and wives, mothers and fathers, will be seen to be wholly interchangeable social constructs, as gender would no longer matter."
Media release Don't Mess with Marriage (ACBC)