The idea is so clearly flawed both in principle and in practice it’s absurd. In principle, the idea that government should promote one kind of happy stable relationship over another is simply offensive. It actively discriminates against widows, single parents, couples where both choose to (or must) work, and parents who choose not to marry.

In practice, there is no evidence – once you control for income, education and whether the kids were planned – that the relationship status has any bearing on child outcomes. Only one-third of couples who would benefit from the tax allowance have kids, and only 17 per cent have kids under five. And of course, it would give absolutely no help to the one in four kids who grow up in single parent families.

Even if the government did want to incentivise marriage, it’s unlikely that three quid a week would make the difference to convince a couple in love to tie the knot, or a keep an unhappy marriage from breaking down.

Nick Clegg – who branded the idea as ‘a throwback to the Edwardian era’ and ‘patronising drivel’ during the election campaign – isn’t stupid. He knows all this. But despite the fact that he won a concession during the coalition talks that his MPs wouldn’t have to vote for it, when he launched the report yesterday, he shied away from criticising the policy in his speech. Hardly a bold move from the man who last week was telling us how his new strategy would be to air differences between the Liberal Democrats and the Conservatives. His cop out gives yet another example of his inability to stand up to Cameron and his lack of power in the coalition

But perhaps more disturbingly, the marriage tax allowance also tells us about Cameron’s playmakers.

David Cameron isn’t stupid either. He also knows all this. And I suspect that during his own whirlwind romance last spring he was been desperate to use his marriage of convenience as an excuse to ditch the marriage tax allowance along with some of the other intolerant ideas in his manifesto. The coalition was the perfect excuse.

But he didn’t. He couldn’t. And that tells us the extent of the nasty party’s grip on the prime minister.

Josie Cluer coordinates the Don’t Judge My Family Campaign – www.dontjudgemyfamily.com