Lincoln’s Conservative MP has denied a constituent’s request for him to support marriage equality for gay and straight couples and told her that in his opinion, gays have ‘exhausted the cause of equal rights’.

Karl McCartney was writing in response to a request from a constituent which used text drafted by the British Humanist Association. He claimed its arguments “could end up” in polygamous and child marriages.

The BHA told PinkNews.co.uk this afternoon Mr McCartney’s assertion “seems not to understand” the argument.

In a letter first reported by Political Scrapbook last night, Mr McCartney cast doubt over “Lynne Featherstone MP’s ‘consultation’ currently taking place. Even someone who had no feelings either way would not consider this a neutral consultation as it presumes an action”.

He told his constituent gay people have “exhausted the cause of equal rights and have now picked on an issue which would possibly only affect a few thousand people every year, whilst also uprooting thousands of years of Christian tradition.

“I applaud the changes made and achieved by the LGBT community in recent decades, but I am concerned that further changes, recent and proposed, will harm the progress that has already been made for little real gain.”

He continued to say that his “conscience dictates” that he will not canvass other MPs for marriage equality and that he is of the view that the “majority of people in same sex relationships are happy with the current arrangements”.

But Mr McCartney, who became the MP for Lincoln in 2010, continued: “If you and your like-minded campaigners were of the opinion that all couples should be able to have civil partnerships, then you might have more credence, but you are not.

“Nor are you advocating multi-partnership marriages, nor a reduction in the age of permitted marriage. But if one takes your ‘arguments’ to any conclusion that is where we could end up.”

“Furthermore, I support the right of any Christian in our Christian country to support the long held belief that a religious marriage is one between an individual man and an individual woman over the age of consent: 16 years of age (18 without parental permission).

“Further, to cite divorce having been in existence longer than Christianity has and to use it as an argument for your point of view is, in my opinion, a misrepresentation of those facts to aid flawed arguments.”

The British Humanist Association-drafted email sent by the constituent, seen by PinkNews.co.uk and reproduced below does not in fact assert that divorce existed before Christianity, but that it was introduced with the formation of the Church of England.

Mr McCartney ended his letter by drawing attention to the Coalition for Marriage “and its 425 000 plus signatories – the largest petition since the General Election – which would indicate that many of the general public in our Country are not on your side in this regard”.

PinkNews.co.uk has been unable to reach Mr McCartney to ascertain which part or parts of the BHA’s argument for marriage equality prompted his fears, but his office confirmed the authenticity of the letter.

Andrew Copson, Chief Executive of the British Humanist Association, told PinkNews.co.uk: “Civil marriage as an institution is a comparatively recent thing, and is based around the willing consent of two people. In other areas of life and law we have removed legal discriminations against lesbian and gay people, based on the fact that such discriminations are morally unacceptable.

“If marriage is an institution based on the free consent of two loving individuals, and if we accept that our law should not discriminate between lesbian and gay people, then there is a clear case for same-sex civil marriage.

“To state that these arguments lead to child marriage and multi-partnerships seems not to understand them.”

The template letter, which the BHA suggests people to send to their MPs, says: “I am writing as your constituent to raise my support for same-sex civil marriage. I am concerned that, at present, the law discriminates against LGBT couples, who are being denied the rights enjoyed by heterosexual couples.

“This is a matter of equal treatment of citizens before the law and, as such, I ask that you sign the Coalition for Equal Marriage’s petition [www.c4em.org.uk] which states: ‘I support the right of two people in love to get married, regardless of gender. It’s only fair.’

“Furthermore, I ask that you canvas Ministers for their support on the equalisation of marriage rights.

“Around the world same-sex marriage has already legalised in many countries and states, including Argentina, Belgium, Canada, Iceland, Mexico City, the Netherlands, Norway, Portugal, South Africa, Spain, Sweden, and six US States including New York.

“Successive polls have demonstrated widespread support for same-sex marriage in the UK as well, both from the LGBT community and society more generally. For instance, a recent ICM poll for the Sunday Telegraph demonstrated that 45% of people were in favour of same-sex marriage, while just 36% were opposed.

“Such is the support for same-sex marriage, that many religious groups, in stark opposition to the edicts of religious leaders, have joined the Coalition for Equal Marriage. These include the Lesbian and Gay Christian Movement, and the Metropolitan Community Church of North London.

“‘Marriage’ has been redefined many times in history. The concept of ‘divorce’, for example, was introduced as comparatively recently as the formation of the Church of England. Clearly, the institution of ‘marriage’ has evolved alongside our society. As such, legislating for same-sex civil marriage would not only reflect our tolerant and open society, but actually serve to enhance it.

“Many thanks for your support.”

The Home Office public consultation asking for views on how to introduce marriage equality for gay and straight couples is running until 14 June.