The Congregation for the Doctrine of the Faith, in the document Considerations regarding Proposals to give Legal Recognition to Unions between Homosexual Persons (June 3, 2003), sets out clearly and in the customary scholarly manner and pastoral sensibility the guidelines for the formation of an authentic Catholic conscience and consequent practice regarding the said matter.
In section IV, relating to the Positions of Catholic Politicians with regard to Legislation in Favour of Homosexual Unions, the document states: “When legislation in favour of the recognition of homosexual unions is proposed for the first time in a legislative assembly, the Catholic law-maker has a moral duty to express his opposition clearly and publicly and to vote against it. To vote in favour of a law so harmful to the common good is gravely immoral”.
A reading of the entire document is highly recommended.