Legalising gay marriage is the "the right thing to do" Culture Secretary Maria Miller has said, amid a rift in the Conservative Party over the plans.
In an article in the Times newspaper, she said being gay was not a good reason to prevent couples marrying.
MPs are to vote on legalising same sex weddings, on Tuesday, but a number of Tory MPs are likely to oppose the bill.
Conservative activists have signed letters voicing both opposition to, and support for, the move.
The Marriage (Same Sex Couples) Bill would enable same-sex couples to get married in both civil and religious ceremonies, where a religious institution had formally consented, in England and Wales.
It would also allow couples who had previously entered into civil partnerships to convert their relationship into a marriage.
'No good reason'
The plans have been strongly backed by Prime Minister David Cameron, but he is facing significant opposition from within his own party.
Reports suggest upwards of 120 Tory MPs could vote against the plans, including some cabinet ministers.
MPs will have a free vote on the issue, meaning they will not be issued orders by party whips, but the measure is expected to get through, with most on the Labour and Lib Dem benches supporting it.
Maria Miller, who is leading the bill through Parliament, defended the proposals, saying marriage as an institution had always evolved and would continue to do so.
"The proposals for change are straightforward - if a couple love each other, then the state should not stop them getting married unless there is good reason - and being gay is not reason enough," she said.
"This bill is about choice. It is about giving those who want to get married the opportunity to do so, while protecting the rights of those who don't agree."
On Sunday, more than 20 current and former constituency chairmen delivered a letter to 10 Downing Street urging the prime minister to delay any parliamentary decision on gay marriage until after the next election.
It warned of "significant damage to the Conservative Party in the run-up to the 2015 election" if the plans enter law.
But supporters of the plan have hit back, with more than 50 Conservative activists, including constituency chairmen, the chair of Conservative Future - the party's section for under 30s - and the chair of the National Conservative Convention - which leads the party's grassroots work - urging MPs to listen to the "wider views" of voters.
In an open letter they warned that opposing gay marriage could "risk alienating the voters we will need in 2015" adding: "To win, the Conservative Party must mount a broad appeal."
Former Conservative minister Sir Peter Bottomley told BBC Radio 4's Today programme arguments against same sex marriage "do not add up" and insisted the party had included the plan in their last manifesto.
"Conservatives believe in fairness. We now understand that to move from a civil partnership to a civil marriage is to allow the words 'I will' at the ceremony," he said. "That is not going to be a big deal."
However, a number of Mr Bottomley's colleagues have opposed the change.
Former Education Minister Tim Loughton said the bill would "set MP against MP" and it was the "wrong time" to introduce such legislation.
A report by think tank ResPublica, to be launched by Conservative MP David Burrowes later, will argue that marriage is "inescapably heterosexual" and the legislation would compromise the meaning of both traditional heterosexual marriage and homosexual partnership.
The new Archbishop of Canterbury, the Most Reverend Justin Welby, who was confirmed in the role on Monday, said he stood by the Church of England's criticism of same sex marriage.
He said: "I have no idea how the vote will go, so I am not going to get into hypothetical questions.
"I stand, as I have always stood over the last few months, with the statement I made at the announcement of my appointment, which is that I support the Church of England's position on this."
In the bill, the Church of England and the Church in Wales will be banned from offering same-sex marriages because of their strong stated opposition, but other religious organisations will be able to "opt in" to holding ceremonies.
Labour leader Ed Miliband said: "I'll be voting for equal marriage for a very simple reason: I don't think that the person you love should determine the rights you have.
"I'll be urging other Labour MPs to maximise our vote so that we can get a decisive majority in the House of Commons, it can then pass through the House of Lords and we can get it into legislation."