The current legal situation around marriage and civil partnership is incoherent.

Since 5th December, under the Marriages and Civil Partnerships (Approved Premises) (Amendment) Regulations 2011, religious bodies in England and Wales have had the right to register same-sex civil partnerships as well as religious weddings, should they wish to do so.

This doesn't mean that same-sex couples have had the right to bang on the door of their local parish church and demand that the vicar conduct a civil union ceremony. Quite the opposite, in fact. The Church of England has made it clear that it has no plans to avail itself of the provisions. No Anglican clergy, however liberal or enthusiastic at the prospect, will be entitled to conduct civil partnerships. And I'd guess the likelihood of the Roman Catholic Bishops Conference repudiating the Pope by signing up to the scheme is vanishingly small. Some believers may have better luck. Liberal synagogues are said to be keen on the measure, the Quakers have already signed up, and the United Reformed Church has promised to consider the matter in July.

It should be clear, then, that the government has bent over backwards to ensure that religious bodies are not compelled to endorse civil partnerships, even to the extent of making life difficult for clergy who, left to their own devices, would like to do so. Yet an Evangelical Tory, Baroness O'Cathain (pronounced "Cahoin"), today forced a House of Lords debate on the issue. Her motion, if passed, would have cancelled the new regulations. In the event, she withdrew it before it reached a vote. Perhaps the tone of the debate had led her to expect a heavy defeat. More likely, in getting the subject debated she had already achieved her aim.

It's not surprising to find Lady O'Cathain's name associated with today's move. A former director of the Barbican Centre ennobled by John Major, she has long had a reputation in the Lords as a campaigner for traditional and religious values. In 2004 she attempted to have civil partnerships excluded from her native Northern Ireland (her intervention, it is rumoured, led to her departure from the board of British Airways after Stonewall threatened a boycott). The same year she opposed the legal recognition of sex changes on the grounds that "the basic proposition of the Bill is mistaken. A man cannot become a woman. A woman cannot become a man".

She also mounted a rearguard attempt in 2008 to save the ancient crime of blasphemy, on the grounds that "as long as there has been a country called England it has been a Christian country, publicly acknowledging the one true God."

Her legal worries about the impact of the new regulations are almost certainly groundless - although to be fair their drafting is sufficiently obscure to allow lawyers to have led one of Britain's most distinguished judges, Baroness Butler-Sloss, to admit during the debate that she had some difficultly understanding them. But in any case, today's debate was mostly about putting down a marker.

What really frightens campaigners of Lady O'Cathain's mindset is the government's desire to legalise full-fledged gay marriage. The current distinction between (exclusively heterosexual) marriage and (exclusively homosexual) civil partnership may be largely an artificial one but it does have significant cultural and religious implications. Many gay couples want to be allowed to call themselves married. Some heterosexual couples would prefer to live without the historical baggage of the word. To offer both types of partnership to every sort of couple seems both liberal and logical.

But not without difficulty. As long as the two are distinct, churches and other religious organisations that offer marriage can legally do so only to heterosexuals; and if they decline to offer civil partnerships to gay people they will not be available to heterosexuals, after all. There will be no discrimination involved, at least not a discrimination that would engage the 2010 Equality Act.

But as soon as marriage is open to all regardless of sexual orientation (and perhaps civil partnership too) this position becomes much harder to sustain. So too does the current distinction between civil weddings and those conducted in a church or other religious building. Something will have to give. Either marriage (and civil partnership) registration will have to become a purely civil matter, with religious bodies free to offer blessings afterwards if they so desire (that being, after all, no concern of the state). Or else, conversely, the state should remove itself from the marriage business entirely and leave it to churches and other voluntary associations to conduct ceremonies and offer pieces of paper to their members that have no more than internal or spiritual relevance. In that case the legal registration of relationships would become a purely administrative matter.

However unfounded Baroness O'Cathain's fears in this particular instance, she's right to note that the current legal situation around marriage and civil partnership is incoherent. The source of the trouble, though, is something of which she presumably approves: the role played by churches and other religious bodies in conducting and registering legally binding marriages. However normal it may seem for people to get married in church, in reality the whole process is a confusion of the proper spheres of religion and the state no less than the presence of bishops in the House of Lords.