The current ban on same-sex marriage is "discriminatory and reinforces homophobia", according to a coalition of faith groups and religious leaders.
The Faith in Marriage group has launched a new campaign outside the Scottish Parliament to lift the ban on religious same-sex marriage.
It follows the launch of a leaflet by Scotland for Marriage, another coalition of faith groups and religious leaders which believes religious same-sex unions will "dismantle marriage which has only ever meant the union of a man and a woman".
Faith in Marriage representatives handed over an open letter to MSPs, demanding the right to conduct same-sex marriages.
It stated: "We believe that lesbian, gay, bisexual and transgender (LGBT) people should be entitled to full equality under the law, and that the current ban on same-sex marriage is discriminatory and reinforces homophobia in society."
Faith in Marriage includes members of the United Reformed Church, the Quakers, the Unitarians, Liberal Judaism, the Humanists, the Iona Community, Buddhists, the Open Episcopal Church, the Metropolitan Community Church and the Pagan Federation. In addition, it has the support of ministers from the Church of Scotland and the Scottish Episcopal Church.
Their joint letter warns that they will "strongly oppose" any attempt to restrict same-sex marriage to civil-only ceremonies. They are seeking assurances from MSPs that any proposed legislation will "protect and extend" freedom of religion and belief by "giving those religious and humanist bodies that do want to conduct same-sex marriage the right to do so".
Rev Scott McKenna, Church of Scotland minister for Mayfield Salisbury Parish in Edinburgh, said: "In opposing equality, churches reinforce homophobia in society and that can lead to pain, low self-respect and, in some cases, violence. In the end this is about people who are on the receiving end of prejudice and are suffering because of that. The cycle needs to be broken."
The Faith in Marriage letter follows the launch of a leaflet earlier by anti-gay marriage group Scotland for Marriage, which will be distributed to almost 300,000 homes before this week's council elections.
Scotland for Marriage includes members of the United Free Church of Scotland, the Church of Scotland, the Evangelical Alliance in Scotland, CARE (Christian Action Research and Education) for Scotland, The Christian Institute, the Fellowship of Independent Evangelical Churches in Scotland and the Christian Brethren in Scotland. In addition, it has the support of Cardinal Keith O'Brien, the head of the Catholic Church in Scotland.