Liberal Democrat MP John Pugh has said that he will vote against same-sex marriage legislation, stating there is “a good liberal case against the current legislation”.

Mr Pugh, MP for Southport, wrote in a letter to constituents posted on his website that he had worried about choosing between his Roman Catholic Church and his political party, but said: “My fundamental objection against the government’s proposal is that it achieves none of its objectives and weakens the link between marriage and the family.”

He argues that the legal basis for marriage was founded in child-rearing and that, although not all heterosexual married couples have children, the potential to procreate is a “coherent underlying rationale” for marriage.

By deviating from this rationale, the government will risk inviting further debate and legislation to clarify its position, the MP says.

“[The government] wants to rule out polygamy, a bisexual menage a trios [sic], incestuous relationships etc. It has a view about the sort of sexual relationships it wants to back.” This is based on “a contemporary preference [for same-sex couples] whose arbitrariness is bound at some time to be questioned,” argues Mr Pugh.

He warns that following this, “the government and courts will be further drawn into adjudicating on the personal and sexual life of its citizens”.

“Whatever we feel morally the private and sexual behaviour of consenting individuals is not a matter for the state,” he says.

Mr Pugh also voices his dissatisfaction with the “quadruple lock” protections for religious groups, saying: “No compromise will hold and religious tolerance will be reduced.”

He gives consideration to what he feels to be the main arguments in favour of same-sex marriage, namely the encouragement of committed relationships, an end to discrimination, and the parity of esteem for same-sex couples, but he argues that the risks outweigh all of these.

“Though personally I am tortured by the thought that I might be on the wrong side of history, my inability to identify a truly coherent thread in the government’s proposals and the sheer arbitrariness of the positions they adopt, consoles me with the thought that I might just be at odds with what flatteringly we call’conventional wisdom’,” he concludes.