Pity the prime minister. With most news bleakly austere these days, changing the law to let gay couples marry must have seemed a sure way to spread crowd-pleasing sweetness and light. Countries around the world are giving homosexuals full marriage rights. More than half of British respondents usually tell pollsters they favour gay marriage. Besides, David Cameron truly believes in it, as he told the Conservative Party conference in October 2011.

At issue is how to balance competing rights—to freedom of religious expression and freedom from discrimination on the basis of sexual orientation. This clash last hit the headlines in 2007, when a Catholic adoption agency was required to consider gay couples as adoptive parents, and the religiously minded owners of a small hotel were told they could not refuse to rent a double bedroom to a gay couple. This time, the dispute is over core religious activities, not charitable or commercial services. But once same-sex marriages are permitted, and on religious premises at that, some fear that churches, mosques and temples could be forced into solemnising relationships of which they disapprove profoundly. Are they right to worry?

The government assures them they need not. All same-sex couples will be entitled to civil marriage, not just the civil partnership they have been allowed since 2005. Those who wish to marry religiously may do so, if a church is willing. A “quadruple lock” has been designed to protect churches that are not against suits for discrimination or breaching human rights.

The new law (which should be on the statute books in 2015) will state that no religious organisation or minister can be compelled to marry same-sex couples. There will be a formal “opt-in” system for those that are interested. The Equality Act of 2010 will be amended. The Church of England, whose canon law is intertwined with the law of the land and normally has a duty to marry people, will not, at its own request, splice same-sex couples, unless canon law and legislation are changed.

That still leaves Strasbourg, which has cheerfully overlooked Parliament’s declared wish and told Britain to let at least some convicted prisoners vote. “There’s no way you can stop a couple going to the European Court of Human Rights and arguing that the fact British law does not oblige religious organisations to marry them is a violation of their rights,” says Robert Wintamute, a barrister who teaches at Kings College London. But they won’t get far, he reckons. Others agree. The European Convention is a touch ambiguous. The court does not require governments to let gay couples marry, still less churches.

But debate over the limits of religious freedom is lively. The hotel owners lost in the Court of Appeals in February, but Lady Justice Rafferty had this to say: “It would be unfortunate to replace legal oppression of one community (homosexual couples) with legal oppression of another (those sharing the Appellants’ beliefs)” The Supreme Court has accepted their case.

Strasbourg too has seen action. Four British people who say their employers denied them religious expression—including Nadia Eweida, initially refused permission by British Airways to wear a visible cross—were heard by the court there in September. A judgment may emerge in January. All of which suggests that politicians may enact as many “locks” as they please, but in the end courts hold the keys.