The legalisation of gay marriage in Britain is the "inevitable outcome" of a process of changing attitudes towards marriage, sex and the family that started with the sexual revolutions of the 1960s.
The Catholic Bishop of Portsmouth, Philip Egan, said in a message to the diocese that sexual intercourse had been increasingly separated from married family life and procreation, and increasingly coupled to "pleasure without responsibility".
"Sexual intercourse could now be experienced outside marriage, and thus, in time, take on a new meaning in human relationships," he said.
"This has led to the 'contraceptive mentality' Pope Paul VI spoke of so prophetically in his 1968 Encyclical Letter Humanae Vitae and to the decline of marriage and now to its redefinition."
He warned that it was only a matter of time before the government moved to widen the definition of marriage even further to include more combinations of partnership.
The bishop went on to say that the Government's "Orwellian" redefinition of marriage presented a "massive" challenge to the Church today in England and Wales.
"What we mean by matrimony, sexual intercourse and family life is no longer what today's world, the government, the NHS and policy-makers understand by marriage, sex and the family," he said.
Bishop Egan warned of a "legal minefield", saying that there would be challenges for Catholic parents raising their children, teachers in Catholic schools, and clergy engaged in pastoral ministry.
"We will certainly need to review our preaching, teaching and school curricula, which henceforth must recognise that our Catholic system of meanings and values is strikingly different from what secular culture now deems normal or acceptable," he said.
Bishop Egan stressed that the Church loved homosexual people and that it had a duty to provide special support to those who wanted freedom from same-sex attraction.
He concluded by calling upon the Church to speak up for traditional marriage "compassionately".
"It remains my hope and prayer that in time, by God's grace and by our gentle love and witness, we will recall society to the path of authentic humanism and thus help everyone hear the call of the Spirit within their hearts to true happiness," he said.
