Gay marriage will be pushed through however heated the debate becomes, deputy prime minister Nick Clegg warned critics of the reform.
The Church of England recently joined a chorus of religious criticism of the proposed move to allow same-sex couples to wed in civil ceremonies.
Tory cabinet minister Owen Paterson is among politicians who have said they will oppose the policy in parliament - with Tory MPs enjoying a free vote on the issue.
But in a video message for the pro-reform Out4Marriage campaign website, Mr Clegg said: "We brought forward our proposals; they have provoked a heated debate.
"But these are proposals about when and how to open up civil marriage to gay and lesbian couples. It's not a matter of 'if' anymore."
He declared: "I've always been very clear on this: love is the same, straight or gay, so the civil institution should be the same too.
"I fought for equal marriage before I was in government, and I'm even more committed to making it happen now - as a Liberal Democrat and as deputy prime minister."
In a direct message to anyone "worried" about the strident opposition to the change, the Liberal Democrat leader added: "Whether you are gay, lesbian, bisexual, transsexual or straight, your freedom to love who you choose is a fundamental right in a liberal society and you will always have our support. That's why I'm Out4Marriage."
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