I'm not sure whether Cardinal Keith O'Brien, the head of the Catholic Church in Scotland, took any PR advice before writing an article comparing homosexuality to slavery. If he did, he probably ignored it given his track record of publicity seeking.
The Catholic Church regards homosexuality as a grave sin, presumably extending a person's stay in Purgatory almost indefintely. Catholic peers led the opposition in the House of Lords to legislation allowing churches to celebrate civil partnerships if they want to. So far, only nice, liberal Christians like the Quakers and Unitarians have taken up the offer. The Church of England unsurprsingly can't make its mind up. Having a less centralised structure than the Catholic Church (although it's hard to see how it could have a more centralised one) it will probably leave the decision to individual vicars. The Church of England would also have probably avoided using the term "slavery" given its relatively recent history of slave-owning.
O'Brien sees civil partnerships as a form of slavery which is no less morally harmful because those who enter them do so by choice, unlike the millions of Africans shipped across the Atlantic. The interesting question is by what right O'Brien and his fellow cardinals think they have the right to police the morals of those outside the Catholic Church, especially when they now police those inside the Church pretty loosely, turning a blind eyes to their communicants taking advantages of scientific advances like contraception and IVF that they similarly rallied against as immoral when they were introduced.
If O'Brien wanted to make a moral statement against slavery, he could of course have spoken out against this.