While some support the introduction of civil partnerships for heterosexuals, others warn of a "dark" and "cynical" attempt by Tory MPs to destroy the bill.
The Defence Secretary says civil partnerships make the introduction of equal marriage unnecessary, but he didn't vote for them in 2004.
The advice, given to MPs today, also refutes suggestions that unwilling clergy might be forced by human rights law to marry same-sex couples.
Whatever the change, all Tories want it to be harder and faster. That is not something the Prime Minister can deliver in a coalition.
Twenty two Labour MPs and four Liberal Democrat MPs voted against the equal marriage bill last night.
The bill to allow equal marriage rights for gay couples moves a step closer to becoming law - but the majority of Tory MPs vote against it or abstain.
136 Conservative MPs voted against the bill, with 127 voting in favour.
Conservative MPs use the existence of civil partnerships as an argument against gay marriage but a significant number voted against them in 2004.
Its opponents may well have a darker motivation than they admit.