THE vote on same sex marriage was a free vote meaning that MPs are allowed to vote with their conscience.
However, because Prime Minister David Cameron has made it such a key issue in his leadership a large number of Tory MPs voting against is in effect a rebellion against him.
But Mr Cameron has faced several other embarrassing rebellions where he has not been able to rely on Labour votes to win the day.
His most troubled area has been Europe where he has suffered two high profile rebellions.
The first time in October 2011 was seen as a watershed when Mr Cameron appeared to lose control of his backbenchers when 81 of them voted for an in/ out referendum on the EU despite orders from the government whips to vote the other way.
The government won the day with the support of Labour, but the rebellion set Mr Cameron on a path to a point where he eventually had to say he would introduce an in/ out referendum by 2017.
However, Labour then joined with 53 Tory backbench rebels to vote to call on the government to negotiate a reduction the EU budget inflicting a humiliating defeat on Mr Cameron ahead of some difficult talks.
The biggest Tory rebellion also had consequences when 91 backbenchers joined Labour in effectively blocking the House of Lords reform bill. They voted against a programme motionm which would have allowed the government to control the debating time and meant if ministers had pressed ahead with it the bill could have been talked out.
Lib Dem Deputy Prime Minister Nick Clegg was so angry about the rebellion that he ordered his MPs to vote against the boundary changes his party had agreed with the Tories.
This meant that the Conservatives in effect lost 20 new safe seats at the next election and made the prospect of aLabour victory much greater.