From the time of the Second Vatican Council until today, various Documents of the Magisterium, and especially the Catechism of the Catholic Church, have confirmed the teaching of the Church on homosexuality. The Catechism distinguishes between homosexual acts and homosexual tendencies.
Regarding acts, it teaches that Sacred Scripture presents them as grave sins. The Tradition has constantly considered them as intrinsically immoral and contrary to the natural law. Consequently, under no circumstance can they be approved.
Deep-seated homosexual tendencies, which are found in a number of men and women, are also objectively disordered and, for those same people, often constitute a trial. Such persons must be accepted with respect and sensitivity. Every sign of unjust discrimination in their regard should be avoided. They are called to fulfil God’s will in their lives and to unite to the sacrifice of the Lord’s Cross the difficulties they may encounter.
In the light of such teaching, this Dicastery, in accord with the Congregation for Divine Worship and the Discipline of the Sacraments, believes it necessary to state clearly that the Church, while profoundly respecting the persons in question, cannot admit to the seminary or to holy orders those who practise homosexuality, present deep-seated homosexual tendencies or support the so-called “gay culture”.
Such persons, in fact, find themselves in a situation that gravely hinders them from relating correctly to men and women. One must in no way overlook the negative consequences that can derive from the ordination of persons with deep-seated homosexual tendencies.
Different, however, would be the case in which one were dealing with homosexual tendencies that were only the expression of a transitory problem – for example, that of an adolescence not yet superseded. Nevertheless, such tendencies must be clearly overcome at least three years before ordination to the diaconate. — from “Concerning the Criteria for the Discernment of Vocations with regard to Persons with Homosexual Tendencies in view of their Admission to the Seminary and to Holy Orders” (2005)