One of the points to civil partnerships, or at least a point made during the political debate that preceded legislation permitting civil partnerships in the UK, is that they are not marriages. That leaves open the possibility that a Catholic might be supportive of the legislation that gives the opportunity to cohabiting same-sex couples to have the same civil and material rights with respect to each other (inheritance, partners pension benefits, tenancy succession and the like) as married couples whilst at the same time not endorsing the same-sex sexual activity, the same-sex union that goes alongside it. But does that really work?
From the point of view of someone who is a Catholic, could they legitimately enter into a civil partnership on the basis that it is just a mechanism in law that assures their civil and material rights? There might be some degree of analogy to a married person who divorces their husband or wife and then does not re-marry - the divorce establishes a framework in which the civil and material good of those affected can be secured though, from the point of view of Catholic belief, the couple are still married. But these two situations are not symmetric. After the civil partnership, the same-sex couple will live together, quite possibly continue to live together; and it would be the case that same-sex union/sexual activity is reasonably presumed to take place (unless there is evidence to the contrary). The public nature of the civil partnership means that this behaviour contrary to the moral teaching of the Church is a matter of public record, just as a second marriage after divorce would be a matter of public record.
Should a Catholic express support for legislative provision for same-sex couples, that is, for civil partnerships as they presently exists in the UK? If they are to do so, a very careful distinction needs to be made clear. The support is not for the same-sex union. The support is for the natural justice in civil and material provision for people who have shared a common life over a period of time. [This is what I have understood to be the position expressed by Archbishop Nichols', and it should not be argued that it is against Catholic teaching.] The difficulty is that, while the Catholic might make this careful distinction, those who promoted and continue to promote civil partnerships do not. For them, it is about promoting the same-sex union as well as the civil and material justice and, with some benefit of hindsight in relation to the present advocacy of gay marriage, simply a step on the way to treating same-sex unions on identical terms to opposite-sex married unions. It might well be a wiser prudential judgement in these circumstances to straightforwardly express opposition to the legislation for civil partnerships.
What can be done to alleviate the element of divided witness arising over the question of civil partnerships? The Bishops Conference of England and Wales could promulgate legislation for the dioceses of our countries according to which Catholics who have entered into civil partnerships are not permitted to receive Holy Communion. Priests should then be able with due prudence and charity - it would be their part in alleviating the divided witness - to refuse Communion to those they know are in civil partnerships. The situation with regard to Catholics in civil partnerships would then be the same as those who are divorced and re-married. In neither case is it a question of discrimination, since those affected are still able to take part in the life of the Church, but one of being consistent in witness to Catholic moral teaching and its significance for ecclesial communion.