In the case of Gas and Dubois v France 25951/07 the European Court of Human Rights considered the issue of whether there was any right to same sex marriage under the European Convention on Human Rights and decided that there was none. Unfortunately the judgment is only available in French "Zut Alors !" however "La Garde meurt mais ne se rend pas!" so I will take the chance of commenting nevertheless.
In the Gas case the Court reaffirmed its earlier decision in Schalk and Kopf v. Austria 30141/04 that there is no obligation under the Convention for States to legalise same sex marriage or indeed to legalise same sex civil partnerships. However it also reaffirmed that if a member State did decide to legalise same sex marriage then they had to ensure that it was provided on exactly the same basis as heterosexual marriage. This has particular relevance for the UK in view of the current consultation the UK and Scottish Governments are running on the possible legalisation of same sex marriage.
In the UK consultation the Government states that the legalisation of same sex marriage would
"make no changes to religious marriages. This will continue to only be legally possible between a man and a woman"
but this assurance is completely at odds with the European Courts decision in both the Schalk and Gas cases.
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.