Opponents can't have it both ways. Either there is no demand for civil partnerships for straight couples or it will be very expensive. But it can't be both.

You can spot the government panic a mile off. Terrified that the amendment being voted on in the Commons today will derail gay marriage legislation, they are throwing everything but the kitchen sink at it.

Most Labour and Liberal Democrat MPs believe in allowing straight couples to have civil partnerships at the same time as allowing gay couples to marry. Tory MPs – up to 100 of which may support the amendment today – are late converts. Or rather, they are not converts at all. They are dinosaurs prepared to vote for anything in a bid to delay or even destroy the gay marriage bill.

Equalities minister Maria Miller has come out with guns blazing. Her department estimates a £4 billion price tag in access to public sector pensions if straight couples can have civil partnerships. Or they do now, at least. A week ago the price tag was £3 billion. It is growing by the day. One might almost suspect they are picking figures out of thin air. They are not. They are constructing numbers out of the most exaggerated estimates available.

Miller's figures are based on the idea that all 2.8 million of Britain's cohabiting couples will have been desperate to have a civil partnership. That seems unlikely.

The Church of England, which also opposes the move, has a more realistic position. "We remain unconvinced that the introduction of such an option would satisfy a genuine and widespread public need, other than for those who pursue 'equality' as an abstract concept," a recent position paper read. "There has been little public evidence to suggest that significant numbers of opposite-sex couples who choose not to marry would opt instead for a civil partnership."

The actual number of straight couples opting for a civil partnership will indeed be very low. It may be popular on the further reaches of the feminist movement, which views marriage as a patriarchal institution. But for the majority of unmarried straight couples, it will make little difference. Most stay unmarried because they do not believe that a public statement of being a couple is necessary to the proper functioning of love and loyalty. Others stay unmarried because they find the entire tradition weird and without purpose. Civil partnerships do nothing to address that view. The only thing that addresses it is not getting married.

The Church of England is right: this is an issue for those who pursue equality as an abstract concept. I am proud to count myself among their number. This is not a practical matter: it is a matter of principle. It always has been. Gay couples already had the rights of married couples under civil partnerships. Gay marriage followed from the principle that people should be equal in their options, not just in their rights.

Gone are the days of 'separate but equal'. Our social and constitutional arrangements must be identical for all our citizens regardless of race, sexuality, gender or any other distinction. No-one should have access to any product or right on the basis of their identity.

These arguments have been made throughout the debate on gay marriage and the government has never addressed them. It only does so now because Tory MPs have become so desperate they will back them in order to delay the gay marriage bill. Doing so has put the fear into many gay rights campaigners. Stonewall says it is "anxious about anything that could delay this much needed change in the law". You'll excuse me for being partially deaf when it comes to Stonewall's view. It refused to back demands for gay marriage when they first arose seriously at a Liberal Democrat conference after the general election. It eventually did so reluctantly, for fear of being left behind by the debate. It's a bit late now for them to enter the debate, again in a predictably conservative way.

It is possible Labour fears are justified – that the government will take the opportunity of delay to drop the bill altogether, quietly, before it causes any more trouble with its backbenchers. But principles of equality should not be contingent on the mania gripping the Tory party. Ministers are saying there could be a two-year delay on the bill due to the change. This was entirely unnecessary if – as should have been obvious from the start – the rights designated to gay and straight couples needed to be equal both ways. In any case, it seems quite mad that arrangements which already exist for one set of people should take two years to be applied to another set of people.

But worst of all, the government's arguments against straight civil partnerships directly contradict the arguments for gay marriage. People must be allowed to conduct their lives as they wish. It is not for the government to say whether there is sufficient demand for something, or how expensive it is. How expensive does something have to become before money overrules equality?

If Tory MPs don't believe in gay marriage, then they shouldn't have one. If the government doesn't believe in civil partnerships for straight couples then they shouldn't get one. But it is not our business to interfere in the choices of those who do.

The truth about civil partnerships for straight couples is precisely the opposite of the government's contradictory arguments. If they prove popular, they will have been worth the money. If they do not, it will be a cost-free way of ensuring proper equality.

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