In the UK, the state offers civil marriage to opposite-sex couples, and civil partnerships to same-sex couples. Civil partnerships cannot contain the language of marriage, even though some religious organisations support same-sex marriages. The Equal Love campaign is seeking to remove this legal barrier.
When I married, I realised the unequal nature of this legislation. We deliberately chose not to have a secular wedding, and I am aware that some of my friends (opposite-sex couples) would have preferred to have a civil partnership. I consider it particularly unfair that same-sex couples are prohibited by the state from having a wedding in a religious setting. If a religious organisation wishes to hold a same-sex wedding, it should not be the role of the state to prevent it. The state should not police religious organisations in this manner, particularly those which are not part of the official state religion*.
I’d go further than the Equal Love campaign, in that I would support the state removing itself from marriage altogether. Perhaps we should let secular venues offer civil partnerships to both same-sex and opposite-sex couples and let religious organisations offer marriages to opposite-sex or same-sex couples depending on the individual place of worship.
(* Of course, faith and the state have a formalised relationship, as the Queen is both the Head of the Church and the Head of State. Yet this does not make the UK actively theocratic; the layers of historic legislation and the distance between crown and politics keeps the official state religion unobtrusive.)