Theresa May has announced that the government will extend civil partnerships to opposite-sex couples as an alternative to marriage. This is a bad idea that will damage our society and I will explain why.

Civil partnerships were introduced by Tony Blair in 2005 to let same-sex couples legally formalise their relationships with exactly the same rights as marriage, but – in order to reduce public opposition to such a dramatic change – not called marriage. That lasted nine years until, after much further debate, Same-Sex Marriage was introduced in 2014. This didn't change the legal rights same-sex couples had – by law civil partnerships already had all the rights marriages have – but for the first time it allowed same-sex couples to be formally part of the same legally and culturally recognised identity. This was considered really important, and argued for and celebrated by many, including myself.

This was important, and was fought over, because marriage is important, and because people argued that 'separate but equal' status with civil partnerships still meant same-sex couples were effectively in a second-rate, separated institution, kept apart from participating with the rest of society.

They were right. Marriage is important, because it is the public statement and recognition of the binding together of two individuals not related by blood to create a new family, with all the commitment and sacrifice, love and immediate concern that should mean. That is why two married people are recognised in law as partially one unit, 'one flesh'. Encouraging commitment is good, it gives people the security and stability they need to build their lives and families.

Of course marriages can fail, and that is why we sadly do need to have divorce, though even the most necessary divorce almost always still involves painful, heartbreaking consequences for years. And marriage isn't for everyone – in fact I'd be the first to encourage people not to get married if they don't feel they can make the commitment. If people don't want that commitment they can cohabit, as many do now.

Introducing civil partnerships for opposite-sex couples will not help this situation and will undermine it. Opposite-sex civil partnerships cannot do what the tiny of number of committed campaigners want them to do. They cannot provide a half-way house between being single and married with less commitment involved, because they are legally identical to marriage, and require the same process to get divorced.

And it would be bad if they did. The commitment involved in marriage is one of the fundamental points of marriage. It is a good thing, and is something we should be enabling and encouraging, not trying to find ways to get around. We will not benefit families, individuals, or society by trying to water it down even more.

Even more, introducing opposite-sex civil partnerships will undermine the shared nature of marriage across society. Marriage is positive because it is a shared experience that all people can aspire to and access. Shared, common institutions, experiences, and loyalties are what unite people, make them willing to recognise and sacrifice for each other, and make society strong. That applies whether it's the Royal Family, the National Anthem, the NHS, tea, queuing, Team GB, or whatever. Same-Sex Marriage campaigners understood that, which is why it was important same-sex couples were included in the same institution. To now encourage separation and division in this vital social experience is divisive and pointless.

It is also something we do with no other legal institution. There are no examples of creating a parallel, identical legal structure for people who claim vague objections to the historical associations of a common concept. Civil partnership campaigners say they find 'marriage' patriarchal and oppressive because history. Well, I might equally find taxes oppressive and feudal because history; or laws, or the concept of a 'government' in general. But nobody in the world is suggesting I should get a totally parallel system of laws, taxes, or government with new, clean progressive names. This basic argument for these new civil partnerships is bogus.

Neither is there any public support for this idea. In 2014 when same-sex marriage was introduced the government did a consultation on what should happen to civil partnerships. 10,000 people responded and over 75 per cent were against extending civil partnerships to opposite-sex couples. There was more support for abolishing civil partnerships entirely straight away than extending them. Since same-sex marriage was introduced new civil partnerships have fallen by 85 per cent, and the total number already existing has fallen every year as people convert their civil partnerships to marriages.

Civil partnerships were a good compromise in 2005 that became obsolete in 2014, and we know people are abandoning them in droves. Now same-sex marriage is embedded and successful we should just close civil partnerships to new applicants, and re-unite marriage as one positive, beautiful social commitment, open to everyone on an equal basis.

Stephen Wigmore has a PhD in Philosophy and Ethics, he writes occasionally at stephenwigmore.com and can be found on Twitter @stephen_wigmore.