Ireland's Church said it would be "a grave injustice" if gay and lesbian couples were granted equality in marriage as it began an uphill battle to persuade voters to reject same sex marriage in a referendum next year.
Ireland will hold the vote just over two decades after the once stridently Catholic country legalised homosexuality and as the once dominant role of Catholicism fades amid revelations of rape and beatings by priests and members of religious orders.
With a recent poll showing 67 percent support for enshrining same-sex marriage in the constitution and 20 percent opposed, Ireland's Catholic bishops launched a 15-page pamphlet setting out its position.
It mirrored a document issued at the end of a synod, of some 200 Roman Catholic bishops from around the world in October which dropped parts of a document that had talked more positively of homosexuals than ever before.
"To put any other view of unions on the same level as Christian marriage would be disservice to society rather than a service," Bishop Liam MacDaid told a news conference.
"In a same sex union, children would be deprived of what a man and woman can give to children in a stable marriage."
A series of investigations into clerical sex abuse have rocked the authority of the Church in Ireland, revealing a state-abetted cover-up at Catholic-run institutions that were labeled places of fear and neglect in a 2009 official report
The Catholic Church helped organise some of the largest protests in decades in France last year to oppose legalisation of gay marriage.
Ireland recognised the legal rights of same-sex couples for the first time in 2009. The move toward further rights follows a government decision last year to allow limited access to abortion that led to large protests from both sides of the debate.
The Church's launch came a day after Irish Prime Minister Enda Kenny was pictured in one of Dublin's main gay bars at an event held by his party's lesbian, gay, bisexual and transgender (LGBT) society.
Kenny had to expel five of his 76-member conservative Fine Gael party for voting against last year's abortion bill, having lost one deputy over economic austerity measures.
"The Taoiseach (Prime Minister) in a gay bar is a first," renowned Irish drag queen Panti Bliss, owner of Pantibar, the bar Kenny visited, wrote on its Facebook page.
"Only a few years ago a Taoiseach wouldn't have dared, so it shows how times have changed."