Allowing same-sex couples to marry will create a "fairer place to live", the Commons has heard ahead of a crunch vote on the divisive issue.
Culture Secretary Maria Miller insisted the Government's proposals would protect freedom of faith and extend equality to gay couples.
But in an indication of the way the issue has split the Conservative Party, one former coalition minister said the Government had no mandate for such a "massive social and cultural change".
Former defence minister Sir Gerald Howarth told her it was not in the Conservative Party's manifesto and Prime Minister David Cameron had said he had "no plans" to introduce the measure.
Scores of Tory MPs are expected to oppose the measure in a free vote on the Marriage (Same Sex Couples) Bill's second reading.
There is also anger about the decision to whip a vote on the timetable for the Bill, with many backbenchers believing that the whole House of Commons should be able to take part in line-by-line scrutiny rather than just a committee of MPs.
The DUP's deputy leader Nigel Dodds said the proposals, which cover England and Wales, were about "low political calculation" aimed at "detoxifying" the Tory Party.
But Mrs Miller said: "We are doing this very clearly as an important part of the way we can make this country a fairer place to live."
She said: "What marriage offers us all is a lifelong partner to share our journey, a loving, stable relationship to strengthen us and a mutual support throughout our lives.
"I believe that this is something that should be embraced by more couples. The depth of feeling, love and commitment is no different between same-sex couples than opposite-sex couples. This Bill enables society to recognise that commitment in the same way too, through marriage."