Christian denominations have not always found it easy to agree about things. But when it comes to sexual ethics, they have been remarkably unified: God made us male and female with the intention that one man should marry one woman, and outside of that lifelong bond sex is to be abstained from.

It is a profoundly countercultural, and powerfully life-giving, vision of human life: one in which, with God's help, loving commitment trumps individual satisfaction, self-control conquers self-indulgence, children are loved and nurtured by their natural father and mother, women are honoured, the vulnerable are protected.

Eastern Orthodox and Coptic, Roman Catholic and Protestant, Presbyterian and Pentecostal may differ profoundly on other things, but on this they have all agreed. It is only in the last few decades that a small number of mainly Western denominations have diverged from this ecumenical consensus, and they have not carried the vast majority with them.

So when the government issued its consultation on banning so-called 'conversion therapy', Christians across these denominations noticed a problem. Not that anyone (that I'm aware of, anyway) has any interest in defending any kind of 'gay cure'; that was never a Christian idea, and such pseudo-therapies when tried have ranged from deeply distressing to wickedly immoral.

But the legislation as proposed would apparently ban something totally different. It would make it illegal in some cases to teach people, or to help people, to follow the standard, mainstream Christian teaching on marriage. Whether through ignorance of Christianity or merely through sloppy drafting is irrelevant; probably it was both. What matters was that this would be a flagrant breach of Christians' right to manifest their religion.

As a result, I and a group of church leaders from across the denominations, most of whom had never met before, came together to draft a letter to Liz Truss, whose department is handling the legislation. We explained that what was proposed could mean priests, pastors, and even Christian parents being sent to prison for up to five years. We also made clear that in such a scenario we would have to, with great sadness, carry on doing our Christian duty whatever the personal cost to us.

Unsurprisingly, the letter gathered wide support from leaders of churches, across the denominations. 2,546 Ministers and Pastoral Workers signed in a few days. We presented the letter at Downing Street two weeks ago, and received apologies and reassurances from the Government Equalities Office that orthodox Christian ministers would not be criminalised for teaching and pastoring according to standard Christian ethics. We wait to see whether that promise will be delivered, of course, and will watch draft legislation carefully. But apart from that, there the story should have ended.

But, bizarrely, it did not. Five of the letter's authors and over 700 of its signatories were from within the Church of England. This was not surprising, given that the official Anglican position on sexual ethics is the standard Christian one I've already outlined. It has of course for several decades had a vocal faction campaigning for this to change; nevertheless, though they have undoubtedly achieved substantial acceptance of dissent from the official position they have not succeeded in persuading their Church to change it. So, many Church of England clergy – those most in agreement with its historic and unchanged position – saw the threat to themselves and added their signatures.

Yet six days after we presented our letter, the Bishop of Dorchester released a statement in which he said that our letter 'cut across the settled view of the Church of England that coercive conversion therapy is unacceptable and should be banned'. This was bizarre given that we specifically repudiated any kind of coercion. What it sought to defend was the doctrine of marriage, which despite what perhaps the bishop wishes, remains the 'settled view of the Church of England'.

Then, four days later, the Bishop of Whitby wrote publicly to his MP stating that the letter 'does not have a monopoly of Christian opinion, nor do they have good safe practice or scientific evidence on their side... it is [a matter] of public safety and welfare' - implying that we were upholding dangerous and damaging practices. He therefore asked for no protections for religious freedom to be provided. But we spoke of nothing but the doctrine of marriage; and if that is the problem, then the Bishop must consider almost the whole of the church throughout history, including his own denomination, to have been a risk to public safety and welfare.

The following day the Bishop of Sheffield wrote to his clergy saying that our letter had caused 'unnecessary anguish', implied it was a safeguarding risk, and directed them to another letter expressing support for the legislation as proposed. But why people signing a letter affirming what the Church of England itself affirms should cause anguish or be a risk to anyone is a mystery. Meanwhile the possibility of a certain amount of anguish among those clergy facing possible imprisonment did not seem to have occurred to him.

Of course, in a Church as broad as the C of E, it is no surprise that many clergy do not hold to historic Christian ethics and therefore are not threatened by the proposed legislation. But the fact remains that there are plenty who do, and therefore who are so threatened. When they teach it, and apply it, they are only doing their job. What's more, they are the ones who are most faithful to the Church's official teaching. And if the government legislates as it has proposed, and as these three bishops have urged, they will be made criminals and face up to five years in jail.

Which demands the question, why do bishops in the Church of England seem as if they want to see some of their own clergy put behind bars? One would have thought that, even if they disagree theologically with conservative clergy, they would have welcomed an attempt to help keep those who work faithfully under them from being declared lawbreakers.

How are clergy who are not persuaded, understandably, that their denomination should depart from 2,000 consistent years of church teaching, to serve under bishops who might then endorse their incarceration for holding that opinion? Particularly when that is the official teaching of their own Church?

As the Post Office has discovered in the aftermath of their Horizon software disaster, criminalising your own employees for nothing more than faithfully doing their job is a very bad idea.

The situation in the CofE smacks badly of a faction which, having been unable to win by theological argument, is seeking instead to enlist the force of law.

Moreover (being myself a Presbyterian and viewing this from outside the confines of the C of E), how does the C of E expect to relate positively to other denominations – from Roman Catholics to Presbyterians to Pentecostals - when its bishops openly support criminalising those who minister in them?

A law which does not give liberty to teach people of every age, as Christians always have, that sex is designed exclusively for the covenant bond between husband and wife, is a law which would put faithful Christian ministers in almost every denomination in prison. It is hard to believe that the Church of England's bishops want that. It is extremely serious if they do.