Legalizing gay marriage will pave the way for polygamy and even siblings getting wed, the former Archbishop of Canterbury Lord George Carey warns in a new book.

The 77 year-old Lord argues the marriage equality bill will lead to demands for multiple-spouse marriages and weddings between cohabiting sisters, in a paper published by the Civitas think-tank.

The intervention from Lord Carey comes after the House of Commons voted to legalize marriage equality and ahead of a vote on the matter next week in the House of Lords.

Lord Carey, who is leading a fight against the legislation in the House of Lords, argues that gay love and commitment are not enough to warrant marriage – because there is no ‘natural possibility of children’.

In an article entitled ‘Love Is Not Enough’, Lord Carey insists ‘not all relationships are the same’ and that without the possibility of procreation the institution of marriage will be ‘diminished’ and ’emptied of its meaning’, with ‘grave implications’ for the family unit.

‘Once we let go of the exclusivity of a one man-one woman relationship with procreation linking the generations, then why stop there?

‘If it is “about love and commitment” then it is entirely logical to extend marriage to, say, two sisters bringing up children together.

‘If it is merely “about love and commitment” then there is nothing illogical about multiple relationships, such as two women and one man.’

He slammed the UK government, saying it has no moral or democratic right to legalize gay marriage, and describes the legislation as ‘hasty and ill-considered’.

Lord Carey also criticized the UK Home Secretary Theresa May, in particular, saying that her support of the bill demonstrates ‘a wholly inadequate understanding of marriage’, reported the Daily Mail.

The former Archbishop said: ‘Those of us accused of being on the wrong side of history can only plead with the government to respect our concern that extending marriage to same-sex couples is not only unwise, but also sets a dangerous precedent.’

However, Roger Lancaster, professor of anthropology at George Mason University, stated that gay marriage has existed for many centuries and that in fact, current wedding vows actually derive from early Greek Christian same-sex commitment ceremonies.

So ‘same-sex relationships were the very models of ideal heterosexual marriages,’ he said in his contribution to the Civitas book.

Maria Miller, UK Culture Secretary and in charge of passing the new law through the House Commons, said: ‘Much of the strength of marriage lies in its ability to change with the times. As society has changed, so marriage has changed, and become available to an increasingly broad range of people.

‘In the 21st century marriage is an inclusive – not exclusive – institution. It is available to all adults who are prepared to make vows of life-long fidelity and commitment. Except, that is, if you happen to love someone of the same sex. I believe that simply isn’t right.’

In an afterword, the book’s editor and Civitas deputy director Anastasia de Waal, a strong supporter of gay marriage, wondered how far Britain has come in terms of embracing gay rights rather than merely tolerating it.

