However, the Archbishop of Dublin, Diarmuid Martin, said a "pluralist society should find a way to respect the rights of same-sex couples, while at the same time respecting the uniqueness of heterosexual relationships".
Archbishop Martin was speaking to diocesan communications officers at All Hallows', Dublin, on Wednesday, ahead of the referendum on whether the constitution should be changed to allow gay marriage to be legalised.
He also acknowledged that the Church had given "harsh" treatment to gays and lesbians in the past "and in some cases still today".
But the Archbishop claimed the proposed change to Article 41 of the constitution is not simply about extending marriage rights, suggesting it changed the definition of marriage itself.
"For me the fundamental question is about the complementarity of men and women, of male and female, in the nature of humanity.
"This is a philosophical concept which some will easily put aside, but for me it is the fundamental one.
"Why do humans exist as male and female? Is that distinction simply marginal? Is it simply a social construct? I am not saying that men and women do not share equality, but that one can only understand and tease out what that equality means within the concept of complementarity," said Archbishop Martin.
Martin also criticised the attitude of many politicians towards the No side, saying they responded to concerns about the amendment with "broken gramophone-like quick soundbites."
