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.
It was the dramatic advances in gay rights delivered by the Blair government that made the introduction of equal marriage possible.
As the issue of same-sex unions finally turns into a parliamentary debate on both sides of the Channel, it is becoming more and more obvious that the Conservatives and their French counterparts have little to win and a lot to lose.
Faith minister Sayeeda Warsi, who attends cabinet, is the latest figure to raise concerns over the policy.