Willie Rennie, the Scottish Lib Dem leader, urged the First Minister to ignore suggestions from the church that 800,000 Catholics will boycott the Nationalists in future elections if the proposal becomes law.
Cardinal Keith O’Brien, the leader of the Catholic Church in Scotland, said at the weekend that the proposal would “shame” the country and “forfeit” the trust held by the religion’s followers in the SNP administration.
Philip Tartaglia, Bishop of Paisley, repeated that a government that legalised gay marriage did not “deserve” the support of the Catholic community.
Mr Salmond has assiduously courted the Catholic vote, particularly in the West of Scotland, who traditionally voted Labour in droves. His charm offensive contributed to the SNP landslide in May’s Holyrood election.
Mr Rennie said: “I am disappointed that leaders of the Catholic Church in Scotland have threatened the Scottish government over proposals to bring equality to marriage.
“Over six in ten people support same sex marriage, so to threaten politicians with the votes of 800,000 people of the Catholic faith may backfire on the Church.”
He urged the First Minister and the other 128 MSPs to stand for what is “right and fair” and recalled how the parliament voted to overturn a ban on promoting homosexuality in schools a decade ago.
Although gay couples can enter a civil partnership, these are secular and cannot contain any religious element. A 14-week public consultation has recently been launched into allowing gay marriage ceremonies in places of worship.
The proposal has already divided the Nationalists after a series of high-profile party figures condemned John Mason, a practising Baptist and backbench MSP, for suggesting that no one should be “forced” to be involved with same-sex marriages.
Mr Salmond has attempted to alleviate concerns by stating that no religious celebrant will have to conduct a gay marriage if it contradicts their beliefs.
But this has failed to mollify the Catholic Church, which believes it is inevitable the legislation will be watered down or a homosexual couple will successfully challenge the opt-out using equalities laws.
In an article for a Sunday newspaper, Cardinal O’Brien denounced the SNP for being “disingenuous” and for its “weasel words” in claiming that churches would not be obliged to solemnise such weddings.
“If the Scottish government attempt to demolish a universally recognised human right, they will have forfeited the trust which the nation, including many in the Catholic community, have placed in them and their intolerance will shame Scotland in the eyes of the world,” he wrote.
In his submission to the consultation, Rev Tartaglia said: “A government which favours and allows for same sex 'marriage' does wrong. It commits an act of cultural vandalism. Such a government does not deserve the trust which the nation, and including many in the Catholic community, has shown in it.”
A Scottish Executive spokesman said ministers “tend towards the view” that gay marriage should be introduced but no final decision has been made.