However, he went further than ministers in calling for the right to hold same-sex marriages to be extended to faith groups as well.
In a video recorded for the Out4Marriage campaign, he said: "I think, whether you are gay or straight, you should be able to signify your commitment, your love with the term 'marriage', and so the Labour Party provides its wholehearted support for this campaign.
"We will be pushing the Government to get on with the process for legislating for equal marriage, and we'll also be saying to them, where faith groups want to provide that opportunity for gay couples as well as straight couples, they should be able to do so.
"We've come a long way on gay and lesbian rights in the last two decades - civil partnerships, age of consent, a range of other things - but I know there's further to go.
The Government is currently consulting on proposals for civil marriages for same-sex couples in England and Wales, with legislation promised before the next general election in 2015.
The Scottish Government, meanwhile, has said it intends to legislate for both civil and religious marriages for same-sex couples in Scotland.
Benjamin Cohen, of Out4Marriage, welcomed Mr Miliband's support for extending the legislation in England and Wales to cover religious ceremonies as well.
"There are many gay couples of faith and there are many faith groups that embrace gay people and so it's only right for faith groups that wish to conduct same-sex marriages to be allowed to do so under law," he said.
"We need full marriage equality, not a half-step that continues to deny gay couples to right to marry in churches, synagogues and meeting houses that are happy to accommodate them. We hope and trust that the Government will come to the same conclusion as the Labour Party."