The Archbishop of Westminster, the Most Reverend Vincent Nichols, said he appreciates some same-sex couples want to be joined in wedlock but insisted the true nature of marriage is between a man and a woman.
Asked what he would say to a gay Catholic couple who approached him for marriage within the Church, the Archbishop said: "I would want to say to them that I understand their desires, that I understand their experience of love is vitally important in their lives, but I would want to say to them that they are called in my view, in the Church's view, to a very profound friendship in life. I would want them to be respected, but I would want them to have a vision in themselves that what they are called to is not marriage but a very profound and lifelong friendship."
Theresa May, the Home Secretary, and Lynne Featherstone, the equalities minister, unveiled a document setting out how same-sex couples would be allowed to register civil marriages.
But the Archbishop told BBC Newsnight: "Marriage is about bringing difference together. Different sexes, sometimes different families, different tribes. It's been used to bring kingdoms together. It's about bringing difference together, out of which comes a new start and a new life. The gender difference is essential for its creativity and its complementarity."
He said it was "utterly astonishing" the consultation did not make reference to children.
"It is excluding things that are of the very nature of marriage. What society says, I believe, is the best circumstances for conceiving and bringing up children is the partnership between two natural parents. That's why the law is there to protect marriage and that's why the change in the definition of marriage affects everybody. I think it's not in the long run a good idea to change, to shake, that fundamental idea of marriage in which a man and a woman call each other husband and wife."
He said he hoped the consultation would be conducted in a "measured and reasonable" manner.
Nick Herbert, the Home Office minister, said the Archbishop had a "sad misunderstanding" of the nature of homosexual relationships.
"It is akin to a union between a man and a woman in a heterosexual marriage," he said. "What is wrong with wanting to show a commitment towards someone?"
He went on: "I don't seek to dictate to the Archbishop what happens inside his Church, what standards he sets and what he seeks to do. It would be quite wrong for me or the state to do so. But equally I wonder why he should seek to dictate the institution of civil marriage outside of his Church which is not a matter for the Church."
He said same-sex unions would "strengthen the institution" of marriage.
"There have been enormous strides towards equality in our society in the last few years, but there is more to do. The idea that we can rest on this issue is wrong. Symbolically, as well as the merits itself, I think it incredibly important that as a society we say the valuable institution of civil marriage is available to all. This is not affecting, or has anything to do with religious marriage, which is untouched by these proposals."
Peter Bone, a Conservative MP, told the programme the twelve-week consultation was a "sham" and the proposals did not appear in party manifestos.
"If you are going to change something that's 2000 years old put it in your manifesto, see if people vote for you, and then bring it forward in the next Parliament."
He added: "This is not my party; it's the Coalition Government. It's because of a dirty deal with the Liberals behind the scenes."
He said it was "utterly astonishing" the consultation did not make reference to children.
"It is excluding things that are of the very nature of marriage. What society says, I believe, is the best circumstances for conceiving and bringing up children is the partnership between two natural parents. That's why the law is there to protect marriage and that's why the change in the definition of marriage affects everybody. I think it's not in the long run a good idea to change, to shake, that fundamental idea of marriage in which a man and a woman call each other husband and wife."
He said he hoped the consultation would be conducted in a "measured and reasonable" manner.
Nick Herbert, the Home Office minister, said the Archbishop had a "sad misunderstanding" of the nature of homosexual relationships.
"It is akin to a union between a man and a woman in a heterosexual marriage," he said. "What is wrong with wanting to show a commitment towards someone?"
He went on: "I don't seek to dictate to the Archbishop what happens inside his Church, what standards he sets and what he seeks to do. It would be quite wrong for me or the state to do so. But equally I wonder why he should seek to dictate the institution of civil marriage outside of his Church which is not a matter for the Church."
He said same-sex unions would "strengthen the institution" of marriage.
"There have been enormous strides towards equality in our society in the last few years, but there is more to do. The idea that we can rest on this issue is wrong. Symbolically, as well as the merits itself, I think it incredibly important that as a society we say the valuable institution of civil marriage is available to all. This is not affecting, or has anything to do with religious marriage, which is untouched by these proposals."
Peter Bone, a Conservative MP, told the programme the twelve-week consultation was a "sham" and the proposals did not appear in party manifestos.
"If you are going to change something that's 2000 years old put it in your manifesto, see if people vote for you, and then bring it forward in the next Parliament."
He added: "This is not my party; it's the Coalition Government. It's because of a dirty deal with the Liberals behind the scenes."