Ok, so a bit of research & it turns out that our official rules come from the Catechism of the Catholic Church. Apparently, the Compendium of the Catechism of the Catholic Church was published in 2005, and the first edition in English in 2006.
And this is apparently what we believe about homosexuality:
The Catechism devotes a separate section to homosexuality within its explanation of the sixth commandment. Like heterosexual acts outside of marriage, homosexual acts are considered sins. The Church distinguishes between homosexual attractions, which are not considered sinful, and homosexual acts, which are. The Catechism states that they "violate natural law, cannot bring forth life, and do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved." The Church teaches that a homosexual inclination is "objectively disordered" and can be a great trial for the person, who the Church teaches must be "accepted with respect, compassion and sensitivity ... unjust discrimination in their regard should be avoided."
Homosexuals are, according to the Church, "called to chastity". They are instructed to practice the virtues of "self-mastery" that teaches "inner freedom" using the support of friends, prayer and grace found in the sacraments of the Church. These tools are meant to help homosexuals "gradually and resolutely approach Christian perfection", which is a state to which all Christians are called.
So, on that basis, I ask once again how the Catholic Church could do anything other than ask its supporters to campaign against homosexual marriage?