Even people like Peter Bone MP engage in this particular argument against equal marriage. That certainly doesn't make it any more right.
1) Considering marriage equality was in a Conservative manifesto at the time of the 2010 General Election! The Conservatives went on to win the largest number of votes.
2) We live in a representative democracy. Our politicians are not bound to only enact things in their manifesto. If we stuck to that rule, then the Commons would need to be dissolved and an election held every time an issue presented itself that hadn't been in anyone's manifesto!
3) Polling shows, despite what some might have you believe, extremely consistent support for equal marriage in the United Kingdom.
4) Best case scenario for proponents of this argument: Parliament is dissolved. There is a General Election. The Tories may win. In which case there will be a mandate for equal marriage. Or Labour might win. In which case there will be a mandate for equal marriage. Unless UKIP somehow manages to squeak to victory , this argument really has no legs.
I'm sure those who make this argument don't mean it to be, but it is extremely disingenuous.
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