Downing Street has been defending its position on equal marriage ahead of the vote on the issue tonight in the House of Commons.
David Cameron's spokesman says the bill is "all about the extension of marriage" to same-sex couples.
MPs will vote on an amendment, which the bill's supporters say is simply a "wrecking amendment" designed to throw the legislation off course.
The amendment calls for the extension of civil partnerships to heterosexual couples.
Those who have tabled it argue a man and a woman should be able to decide either to get married or have a civil partnership - just as gay couples will be able to - if this legislation becomes law.
The Government has proposed to review the arrangements for civil partnerships after five years.
Number 10 said it was a free vote for MPs "recognising there is a range of views" on this issue.
Deputy Prime Minister Nick Clegg has said the amendment is nothing more than an "ulterior motive" by opponents of equal marriage, adding:
I don’t want anything to interfere with the central purpose of this legislation.