The Conservative group that backs gay marriage is supported by Tories including Boris Johnson, Michael Gove, Transport Secretary Patrick McLoughlin, who is Catholic, Foreign Office minister Alistair Burt, an evangelical Christian, and Scottish Conservatives leader Ruth Davidson.
In a letter to The Sunday Telegraph announcing the formation of the group, the 19 declared: "Marriage should be open to all, regardless of sexuality.
"We recognise that civil partnerships were an important step forward in giving legal recognition to same sex couples.
"But civil partnerships are not marriages, which express a particular and universally understood commitment."
Same-sex marriage will remain illegal in the Church of England, as the Government unveiled its plans to extend marriage to gay couples.
David Cameron is facing a backlash from Tory MPs and religious groups over allowing gay weddings in churches.