Lord Carey has joined forces with the likes of Norman Tebbit and Jeremy Irons in their fight against equal marriage.

The former Archbishop of Canterbury agrees that the law could lead to incest, with “sisters marrying sisters”, as well as encouraging polygamy.

With his thoughts published in a new book by Civitas, The Meaning of Matrimony, Lord Carey argues that if marriage is based solely on ‘love and commitment’, eliminating the condition of procreation, then “it is entirely logical to extend marriage to, say, two sisters bringing up children together”.

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

His opposition to the government’s plans, which go before the House of Lords on Monday, echo the sentiments of former cabinet minister Norman Tebbit, and actor Jeremy Irons, who argued that fathers could marry sons to avoid inheritance tax if the same-sex marriage law was passed.

They all seem to have forgotten that incest is, in fact, illegal.

Meanwhile, gay rights campaigner Peter Thatchell, claims that the new Archbishop of Canterbury, Justin Welby, appears to be in favour of the bill, writing, “My feeling is that Welby would, in his heart, like to support marriage equality but feels bound by centuries of church.”

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