Really interesting opinion lining up with what the Church of England is saying.
What the Government assurance is ignoring is the fact that, in law, there is no difference between “Civil” as opposed to “Religious” marriage both are in law the same thing and merely take place in different premises. Therefore on the basis of the both the Schalk and Gas cases if the Government legalises same-sex marriage then it must legalise it on exactly the same basis as it legalises heterosexual marriage ie the Government will be obliged to permit same-sex marriage on religious premises on exactly the same basis as it permits heterosexual marriage on religious premises.
How this will affect the rights of Churches who are registered for marriage and in particular how it will affect the Church of England and its clergy who are Registrars of Marriage by virtue of their status as Priests of the Established Church is legally very arguable. Certainly a good legal case can be made that any place or person who is registered to perform marriage must be willing to perform same sex marriage on the same basis as they conduct heterosexual marriage since, in law, there will be no difference between the two.
It must also be remembered that in the case of Ladelle v Islington Council  EWCA Civ 1357 the Court of Appeal held that Mrs Ladele’s view of Marriage “the orthodox Christian view that marriage is the union of one man and one woman for life” (para 7) “was not a core part of her religion” (para 52) and therefore if Churches are told that they have to be willing to perform same sex marriage ceremonies they will have little legal ground to resist.
The combined effect of the European Court decision and the Ladele decision seems to be clear. If same sex marriage is legalised in the UK then religious same sex marriage will have to be legalised also. Churches which perform heterosexual marriages will have to be willing to perform same sex marriages and they will have no legal grounds to resist since the (secular) Courts have determined that the “Orthodox Christian view of Marriage” is not a “Core” part of Christian belief.
In the comments there is an interesting clarification.
I’m not a lawyer so I might be wrong but I believe in Spain gay marriage is legal and the RC church has not been forced to perform them or stop performing all marriages. Is it likely a similar thing will happen in the UK?
A fair question. it depends a lot upon the structure and legislation applying to Marriage in Spain.
In many parts of Europe there is a clear legal distinction between Civil registered Marriage and Religious ceremonial Marriage but that legal distinction does not exist in the UK. In addition the UK is fairly unique in Europe in making the European Convention and European Court of Human Rights cases directly applicable in UK law.
Finally the decision by the Court of Appeal that belief in Marriage is not a “core” part of Christian belief is unique to the UK and is not based on European case law however that decision makes the legal position in Britain very different to that in other European countries who would probably accept that it is not the place of secular courts to make that sort of decision or that sort of distinction.
I would be interested if any other lawyers disagree.