The Catholic Church in Australia has advised its followers to call for politicians to resist gay marriage proposals based on the “natural order”.

MPs in Canberra are expected to receive calls from constituents who have been counselled by the Church to stress the importance of biological relationships, rather than protecting marriage as a religious institution.

The Sydney Archdiocese, the Catholic Women’s League and the Knights of the Southern Cross issued an open letter urging Catholics to take action.

The letter states: “Marriage between a man and a woman is not a religious construct but a natural institution found across all cultures and religions. Marriage is a unique kind of sexually complementary union with a natural orientation to life.”

The church has also written that heterosexual couples who are married but infertile have a greater right marriage than homosexuals because “their lovemaking is designed to give life”.

“Allowing two men or two women to marry would involve a fundamental change in our understanding of marriage, from a life-giving and sexually complementary union to a personal, romantic relationship with no true communion or connection to procreation.”

Currently, Australia’s Labor Party has an official policy against gay marriage.

The policy has been questioned recently, and a motion tabled by the Australian Greens party called on representatives to consult their constituents on the issue.

The motion was passed by 73 votes to 72.

Marriage Equality Spokesman Rodney Croome said the church should “should respect our right to marry under civil law – this kind of mutual tolerance is the hallmark of a modern, civilised democracy”.

“Our advice to the people we are encouraging to visit MPs is to highlight the fact that under Australian marriage law people of any faith or no faith can marry, as can people who cannot or do not wish to have children.”