The Coalition honoured Labour's Equality Bill to the full.
It became law.
Same Sex marriage was never any part of it, and it is very misleading of you to claim it as some kind of mandate.
There was no mandate
How could there be?
No-one knew it was about to be pushed through Parliament on Cameron's whim.

No mention in any manifesto, Labour, Tory or Lib-Dem.
No consultation.
No Green Paper.
No mention in Queens Speech on Government intentions.
It came from nowhere.
It was never an issue of any significance before, but now it is suddenly a priority.