A Christian group in the Channel Island of Jersey is preparing a document warning politicians that equal marriage will lead to increased infidelity in heterosexual marriages.

A draft letter from the Jersey Evangelical Alliance, which claims to represent a quarter of churches in Jersey, has been leaked to BBC Radio Jersey. A finalised version is planned to be circulated around members of the States, Jersey’s parliamentary body, ahead of a debate on equal marriage being held next week.

The letter reads: “Rather than extending the benefits of marriage to same-sex couples, redefining marriage would introduce the instabilities and infidelities commonly associated with homosexual relationships into society’s understanding of marriage.”

Marriage is defined in the letter as being “voluntary, heterosexual, monogamous and lifelong”.

It goes on to say: “The States should no more be asked to legislate for same-sex marriage than it should be asked to legislate for forced marriages, polygamous marriages, or temporary contract marriages.”

In the letter, the group, which is chaired by Reverend Mike Taylor, also condemns same-sex parenting, and suggests equal marriage would harm children.

It claims that both parents and teachers would object to schools teaching that “society recognises no distinction between a marriage between a man and a woman, and a marriage between two men or two women”.

Jersey politician and equal marriage campaigner Deputy Sam Mezec has dubbed the letter “sinister”. He told the BBC: “It’s incredibly intolerant and represents the worst of what some secular people see in religious attitudes.”

Responding to the letter, Martin Gavet, Chair of Guernsey-based LGBTQ rights group Liberate, said: “I would like to challenge Reverend Mike Taylor to back up his argument with facts, instead of descending to the politics of the playground. LGBT people have suffered for far too long persecution by certain religious groups and this is a prime example of that hatred.”

The Channel Islands, the two largest of which are Jersey and Guernsey, have lagged behind the UK on LGBT issues.

Guernsey equalised the age of consent in 2010, and its Chief Minister this year pledged to introduce civil unions.

Jersey introduced civil partnerships in 2012. This month, the Trade Union Congress (TUC) urged Jersey to also introduce equal marriage.