SINCE Lord Wolfenden's celebrated report was published, it has occasionally been noted that the love that dared not speak its name has become the love which will not shut up about itself, but surely few would have predicted that half a century later the most vocal advocates of gay marriage would be a Conservative Prime Minister and several of his senior ministers.

Nor, I imagine, would many people have envisaged a situation where the Church of Scotland was embroiled in a row with one of its best-known congregations. The members of St George's Tron, a traditional evangelical stronghold, have chosen to leave the Kirk over its acceptance of gay clergy.

Perhaps these developments were not anticipated because, at first sight, they seem not to reflect the interests of most of those involved. David Cameron may be convinced that endorsing gay marriage is the right thing to do, but it is surprising that it should be high on his list of priorities. We have, after all, legislation which extends the civil rights of married people to those in civil partnerships, and he seems to have picked a rather pointless fight with many people he might expect to be his supporters over a word.

The Kirk, meanwhile, has picked a fight with those of its members who continue to believe something which it has taught for all its existence. Personally, I've always found it odd that some Christians seem obsessed by this particular sin (if you think it is one). I can only conclude that it's convenient to condemn something that doesn't tempt you personally, though I'd think the sanctimony and hypocrisy of such a stance ranks higher in the list of grave sins than homosexual acts.

There is, however, no matter how much it offends modern liberal sensibilities, no getting away from the fact that opposition to homosexual acts is laid down in Scripture and remains the orthodox teaching of the vast majority of Christian churches – to say nothing of many other major religions.

Of course, Scripture proscribes a great many things, such as mixed fibres, shellfish and associating with women who are menstruating, which not even the most severe Christian sects pay a blind bit of attention to. The majority of the population has long since moved on from the view that homosexuality is a perversion which should be criminal – only one member of Wolfenden's panel (James Adair, then Glasgow's procurator-fiscal) did not agree with legalising homosexual acts in private – and it would not be surprising if some religious groups also revised their opinion, as the Kirk, the Quakers and many liberal Anglicans already have.

What is less easy to explain, as it is with the Tory advocates of gay marriage, is why such a huge fuss should be made. In the case of the politicians, David Cameron has two reasons. The first is his assertion that, since marriage, and supporting it, are conservative notions, the same should be true of gay marriage. The second is that he wants to "detoxify" the Tory party's image as intolerant and illiberal.

His opponents, of course, argue that gay marriage is different from straight marriage for a variety of reasons, not all of them plausible. The statement issued by the Church of England, for example, dwelt on the difference between men and women, which echoes the argument from "complementarity" advanced by the Swiss Protestant theologian Eric Fuchs. It's difficult to see that homosexuals have much they can do about that, or that it's a good argument against their having stable and loving relationships.

But if we have civil partnerships to protect rights (such as tenancies and inheritance), and if metropolitan liberals are unlikely to vote Conservative, even if the party puts this legislation through, we're back to the question of why Mr Cameron is so insistent on the matter at a time when one might think he has more pressing things to worry about.

I can only explain it, and the Kirk's outrage with its former congregation, by pointing out that when there is a change in attitude towards anything regarded as falling into the sphere of ethics, it is very often followed by a period of moral inversion. I should make it plain that I mean that phrase to be purely descriptive, and not judgmental: it is that, having changed our minds on an issue of morality, we often seem inclined not merely to allow what was formerly condemned, but to elevate it and at the same time, direct our condemnation at those who oppose the change.

That has certainly happened with this subject. The terms in which homosexuality was discussed in Parliament in the aftermath of Wolfenden's report – and by the people supporting liberalisation, mind you – would probably now get them charged with hate crime. The wave of outrage and public revulsion previously directed at homosexuals is now likely to pour down on those who oppose homosexual equality, or persist in regarding it as morally wrong. Perhaps it is born of society's guilt at having held what it now regards as a wrong opinion.

Many of us may regard that as a good thing, or at any rate an improvement. But even if you don't, the inversion itself is an observable phenomenon. All the same, it creates dangers and problems such as those the Kirk and the Government are having to consider. There is the very real prospect that when something is allowed, the denial of it will later become illicit. Everyone says now that churches and mosques will not be compelled to marry gay people, but in time the consequences of taking a stand against it may prove very serious, as the Tron has already discovered.

Those in favour of marriage equality should equally be alert to this danger, and indeed, one of the most vocal defenders of the right of people to declare their opposition to homosexuality has been the activist Peter Tatchell. He advocates gay marriage (and, logically enough, also offering civil partnerships to straight couples) but also defends the right to free speech of people that he must think of as ignorant bigots. That is a principled and consistent position, reached after a lifetime of campaigning on such matters, as well as his admirable work on other human rights issues.

Mr Tatchell can see that, because these issues matter, they need to be thought about, and everyone's rights protected. By contrast, the Government and the churches seem to be barging ahead without thinking. Even if their intentions are good, that could end up very messily indeed.